Fix StudentTCopula DF validation and harden download tests against outages

StudentTCopula: DegreesOfFreedom setter no longer throws — it sets
_parametersValid via the new ValidateParameters(rho, nu) and constructors follow suit. ValidateParameter now delegates to ValidateParameters so the base Theta setter validates both parameters. Unify the ν minimum on 2 + 1e-10 across ParameterConstraints and SetCopulaParameters; the previous 2.0 + Tools.DoubleMachineEpsilon expression silently rounded to 2.0 in IEEE 754 (ULP at 2.0 is 2^-51, larger than eps = 2^-53), making the optimizer fail at the boundary. Tests updated for the int→double widening and the new minimum.

Test_TimeSeriesDownload: add per-service AvailableAsync probes (CHMN, USGS, GHCN, BOM) memoized via ConcurrentDictionary<string, Lazy<Task<bool>>>; replace
`Online()` in all 31 integration tests so an upstream outage skips cleanly
instead of failing CI. BOM URLs switched to https and 5xx response bodies are now surfaced in the exception so the next outage is self-documenting.

Author: HadenSmith

Numerics/Data/Time Series/Support/TimeSeriesDownload.cs

@@ -942,7 +942,7 @@ public static async Task<TimeSeries> FromABOM(
             else parameterType = "Rainfall";
 
             // Step 1: Get timeseries list to find the appropriate ts_id
-            string tsListUrl = "http://www.bom.gov.au/waterdata/services" +
+            string tsListUrl = "https://www.bom.gov.au/waterdata/services" +
                 $"?service=kisters&type=QueryServices&datasource=0&format=json" +
                 $"&request=getTimeseriesList" +
                 $"&station_no={Uri.EscapeDataString(stationNumber)}" +
@@ -971,7 +971,6 @@ public static async Task<TimeSeries> FromABOM(
                 try
                 {
                     response = await client.GetAsync(tsListUrl);
-                    response.EnsureSuccessStatusCode();
                 }
                 catch (HttpRequestException ex)
                 {
@@ -984,6 +983,15 @@ public static async Task<TimeSeries> FromABOM(
 
                 var listResponse = await response.Content.ReadAsStringAsync();
 
+                // Surface the response body in the exception so transient upstream failures
+                // (KiWIS 5xx with a JSON error payload) are self-documenting.
+                if (!response.IsSuccessStatusCode)
+                {
+                    string snippet = listResponse.Length > 500 ? listResponse.Substring(0, 500) + "…" : listResponse;
+                    throw new Exception(
+                        $"Failed to retrieve time series list from BOM API. Status={(int)response.StatusCode} ({response.StatusCode}). Body={snippet}");
+                }
+
                 // Check for error response from KiWIS
                 if (listResponse.Contains("\"type\":\"error\"") || listResponse.StartsWith("{\"type\":\"error\""))
                 {
@@ -1063,7 +1071,7 @@ public static async Task<TimeSeries> FromABOM(
             }
 
             // Step 2: Get time series values using the ts_id
-            string valuesUrl = "http://www.bom.gov.au/waterdata/services" +
+            string valuesUrl = "https://www.bom.gov.au/waterdata/services" +
                 $"?service=kisters&type=QueryServices&datasource=0&format=json" +
                 $"&request=getTimeseriesValues" +
                 $"&ts_id={tsId}" +
@@ -1094,7 +1102,6 @@ public static async Task<TimeSeries> FromABOM(
                 try
                 {
                     response = await client.GetAsync(valuesUrl);
-                    response.EnsureSuccessStatusCode();
                 }
                 catch (HttpRequestException ex)
                 {
@@ -1107,6 +1114,16 @@ public static async Task<TimeSeries> FromABOM(
 
                 var valuesResponse = await response.Content.ReadAsStringAsync();
 
+                // Surface the response body in the exception so transient upstream failures
+                // (e.g. KiWIS returning 500 with `{"code":"DatasourceError","message":"Error connecting to WDP."}`)
+                // are self-documenting. Without this, EnsureSuccessStatusCode would discard the body.
+                if (!response.IsSuccessStatusCode)
+                {
+                    string snippet = valuesResponse.Length > 500 ? valuesResponse.Substring(0, 500) + "…" : valuesResponse;
+                    throw new Exception(
+                        $"Failed to retrieve time series values from BOM API. Status={(int)response.StatusCode} ({response.StatusCode}). Body={snippet}");
+                }
+
                 // Check for error response
                 if (valuesResponse.Contains("\"type\":\"error\"") || valuesResponse.StartsWith("{\"type\":\"error\""))
                 {

Numerics/Distributions/Bivariate Copulas/StudentTCopula.cs

@@ -75,11 +75,8 @@ public StudentTCopula()
         /// </summary>
         /// <param name="rho">The correlation parameter ρ (rho). Must be in [-1, 1].</param>
         /// <param name="degreesOfFreedom">The degrees of freedom ν (nu). Must be greater than 2.</param>
-        /// <exception cref="ArgumentOutOfRangeException">Thrown when ν ≤ 2.</exception>
         public StudentTCopula(double rho, double degreesOfFreedom)
         {
-            if (degreesOfFreedom <= 2)
-                throw new ArgumentOutOfRangeException(nameof(degreesOfFreedom), "The degrees of freedom must be greater than 2.");
             _nu = degreesOfFreedom;
             Theta = rho;
         }
@@ -91,11 +88,8 @@ public StudentTCopula(double rho, double degreesOfFreedom)
         /// <param name="degreesOfFreedom">The degrees of freedom ν (nu). Must be greater than 2.</param>
         /// <param name="marginalDistributionX">The X marginal distribution.</param>
         /// <param name="marginalDistributionY">The Y marginal distribution.</param>
-        /// <exception cref="ArgumentOutOfRangeException">Thrown when ν ≤ 2.</exception>
         public StudentTCopula(double rho, double degreesOfFreedom, IUnivariateDistribution? marginalDistributionX, IUnivariateDistribution? marginalDistributionY)
         {
-            if (degreesOfFreedom <= 2)
-                throw new ArgumentOutOfRangeException(nameof(degreesOfFreedom), "The degrees of freedom must be greater than 2.");
             _nu = degreesOfFreedom;
             Theta = rho;
             MarginalDistributionX = marginalDistributionX;
@@ -115,14 +109,12 @@ public StudentTCopula(double rho, double degreesOfFreedom, IUnivariateDistributi
         /// <see cref="StudentT"/> and <see cref="MultivariateStudentT"/> accept non-integer
         /// degrees of freedom.
         /// </remarks>
-        /// <exception cref="ArgumentOutOfRangeException">Thrown when the value is ≤ 2.</exception>
         public double DegreesOfFreedom
         {
             get { return _nu; }
             set
             {
-                if (value <= 2)
-                    throw new ArgumentOutOfRangeException(nameof(DegreesOfFreedom), "The degrees of freedom must be greater than 2.");
+                _parametersValid = ValidateParameters(Theta, value, false) is null;
                 _nu = value;
             }
         }
@@ -178,18 +170,40 @@ public override double ThetaMaximum
         }
 
         /// <inheritdoc/>
+        /// <remarks>
+        /// The base <see cref="BivariateCopula.Theta"/> setter calls this method on the new
+        /// rho value; we delegate to <see cref="ValidateParameters(double, double, bool)"/>
+        /// so that <c>_parametersValid</c> reflects the validity of both ρ and the current ν.
+        /// </remarks>
         public override ArgumentOutOfRangeException? ValidateParameter(double parameter, bool throwException)
         {
-            if (parameter < ThetaMinimum)
+            return ValidateParameters(parameter, _nu, throwException);
+        }
+
+        /// <summary>
+        /// Validates the correlation ρ and degrees of freedom ν together. Returns null when
+        /// both parameters are in their valid domains.
+        /// </summary>
+        /// <param name="rho">The correlation parameter ρ. Must be in [<see cref="ThetaMinimum"/>, <see cref="ThetaMaximum"/>].</param>
+        /// <param name="degreesOfFreedom">The degrees of freedom ν. Must be finite and greater than 2.</param>
+        /// <param name="throwException">When true, throws on the first invalid parameter; when false, returns the exception instead.</param>
+        public ArgumentOutOfRangeException? ValidateParameters(double rho, double degreesOfFreedom, bool throwException)
+        {
+            if (rho < ThetaMinimum)
             {
                 if (throwException) throw new ArgumentOutOfRangeException(nameof(Theta), "The correlation parameter ρ (rho) must be greater than " + ThetaMinimum.ToString() + ".");
                 return new ArgumentOutOfRangeException(nameof(Theta), "The correlation parameter ρ (rho) must be greater than " + ThetaMinimum.ToString() + ".");
             }
-            if (parameter > ThetaMaximum)
+            if (rho > ThetaMaximum)
             {
                 if (throwException) throw new ArgumentOutOfRangeException(nameof(Theta), "The correlation parameter ρ (rho) must be less than " + ThetaMaximum.ToString() + ".");
                 return new ArgumentOutOfRangeException(nameof(Theta), "The correlation parameter ρ (rho) must be less than " + ThetaMaximum.ToString() + ".");
             }
+            if (double.IsNaN(degreesOfFreedom) || double.IsInfinity(degreesOfFreedom) || degreesOfFreedom <= 2.0)
+            {
+                if (throwException) throw new ArgumentOutOfRangeException(nameof(DegreesOfFreedom), "The degrees of freedom must be greater than 2.");
+                return new ArgumentOutOfRangeException(nameof(DegreesOfFreedom), "The degrees of freedom must be greater than 2.");
+            }
             return null;
         }
 
@@ -204,12 +218,13 @@ public override double ThetaMaximum
         /// ν is clamped to the valid domain (ν &gt; 2) but otherwise kept continuous — no
         /// integer rounding. MCMC samplers that propose continuous ν values see a smooth
         /// likelihood, avoiding the step-function plateaus produced by previous rounding
-        /// behavior.
+        /// behavior. The clamp uses 2 + 1e-10 because <c>2.0 + Tools.DoubleMachineEpsilon</c>
+        /// rounds back to 2.0 in IEEE 754 (the ULP at 2.0 is 2^-51, larger than ε = 2^-53).
         /// </remarks>
         public override void SetCopulaParameters(double[] parameters)
         {
             Theta = parameters[0];
-            DegreesOfFreedom = Math.Max(2.0 + Tools.DoubleMachineEpsilon, parameters[1]);
+            DegreesOfFreedom = Math.Max(2.0 + 1E-10, parameters[1]);
         }
 
         /// <inheritdoc/>
@@ -226,7 +241,7 @@ public override void SetCopulaParameters(double[] parameters)
             return new double[,]
             {
                 { -1 + Tools.DoubleMachineEpsilon, 1 - Tools.DoubleMachineEpsilon },
-                { 3, 30 }
+                { 2.0 + 1E-10, 30 }
             };
         }