fix: harden unavailable/local chat fallback behavior

- Make UnavailableChatService throw ChatBackendUnavailableException for both completion APIs.
- Use per-entry MemoryCacheEntryOptions in LocalChatService history cache writes.
- Wrap response body-read transport/timeouts as ChatBackendUnavailableException.
- Replace project-wide EXTEXP0001 suppression with targeted pragma around
  RemoveAllResilienceHandlers usage.

Author: BenjaminMichaelis

EssentialCSharp.Chat.Shared/EssentialCSharp.Chat.Common.csproj

@@ -2,9 +2,6 @@
 
   <PropertyGroup>
     <TargetFramework>net10.0</TargetFramework>
-    <!-- Suppress experimental API warning from RemoveAllResilienceHandlers() used to opt out
-         the local AI HttpClient from the global standard resilience handler. -->
-    <NoWarn>$(NoWarn);EXTEXP0001</NoWarn>
   </PropertyGroup>
 
   <ItemGroup>

EssentialCSharp.Chat.Shared/Extensions/ServiceCollectionExtensions.cs

@@ -142,6 +142,7 @@ public static IServiceCollection AddConfiguredChatServices(this IServiceCollecti
                 return services;
             }
 
+#pragma warning disable EXTEXP0001
             services.AddHttpClient(LocalChatHttpClientName, client =>
             {
                 client.BaseAddress = localEndpointUri;
@@ -152,6 +153,7 @@ public static IServiceCollection AddConfiguredChatServices(this IServiceCollecti
             // cut off long local-LLM completions. We set HttpClient.Timeout directly instead.
             // Retries are also wrong for LLM calls (non-idempotent, partial responses).
             .RemoveAllResilienceHandlers();
+#pragma warning restore EXTEXP0001
             services.AddSingleton<IChatCompletionService, LocalChatService>();
             Console.WriteLine("[AI] Selected backend: Local (Ollama/OpenAI-compatible).");
             return services;

EssentialCSharp.Chat.Shared/Services/LocalChatService.cs

@@ -1,4 +1,5 @@
 using System.Linq;
+using System.IO;
 using System.Net.Http.Headers;
 using System.Text;
 using System.Text.Json;
@@ -20,9 +21,6 @@ public partial class LocalChatService : IChatCompletionService, IDisposable
     private readonly IHttpClientFactory _HttpClientFactory;
     private readonly ILogger<LocalChatService> _Logger;
     private readonly MemoryCache _ConversationHistory = new(new MemoryCacheOptions { SizeLimit = MaxConversationEntries });
-    private static readonly MemoryCacheEntryOptions _HistoryEntryOptions = new MemoryCacheEntryOptions()
-        .SetSlidingExpiration(_HistoryTtl)
-        .SetSize(1);
 
     public bool IsAvailable => true;
 
@@ -76,7 +74,23 @@ public void Dispose()
 
         using (response)
         {
-            var body = await response.Content.ReadAsStringAsync(cancellationToken);
+            string body;
+            try
+            {
+                body = await response.Content.ReadAsStringAsync(cancellationToken);
+            }
+            catch (HttpRequestException ex)
+            {
+                throw new ChatBackendUnavailableException("Local AI backend is unavailable while reading response.", ex);
+            }
+            catch (IOException ex)
+            {
+                throw new ChatBackendUnavailableException("Local AI backend connection closed while reading response.", ex);
+            }
+            catch (TaskCanceledException ex) when (!cancellationToken.IsCancellationRequested)
+            {
+                throw new ChatBackendUnavailableException("Local AI backend timed out while reading response.", ex);
+            }
 
             if (!response.IsSuccessStatusCode)
             {
@@ -89,7 +103,10 @@ public void Dispose()
                 var (text, responseId) = ParseResponse(body);
                 history.Add(new LocalChatMessage("user", prompt));
                 history.Add(new LocalChatMessage("assistant", text));
-                _ConversationHistory.Set(responseId, history.TakeLast(MaxConversationMessages).ToList(), _HistoryEntryOptions);
+                var entryOptions = new MemoryCacheEntryOptions()
+                    .SetSlidingExpiration(_HistoryTtl)
+                    .SetSize(1);
+                _ConversationHistory.Set(responseId, history.TakeLast(MaxConversationMessages).ToList(), entryOptions);
                 return (text, responseId);
             }
             catch (Exception ex) when (ex is JsonException || ex is InvalidOperationException || ex is NotSupportedException)

EssentialCSharp.Chat.Shared/Services/UnavailableChatService.cs

@@ -20,11 +20,10 @@ public class UnavailableChatService : IChatCompletionService
         string? endUserId = null,
         CancellationToken cancellationToken = default)
     {
-        return Task.FromResult<(string response, string responseId)>(
-            ("Chat service is unavailable in this environment.", Guid.NewGuid().ToString("N")));
+        throw new ChatBackendUnavailableException("Chat service is unavailable in this environment.");
     }
 
-    public async IAsyncEnumerable<(string text, string? responseId)> GetChatCompletionStream(
+    public IAsyncEnumerable<(string text, string? responseId)> GetChatCompletionStream(
         string prompt,
         string? systemPrompt = null,
         string? previousResponseId = null,
@@ -35,10 +34,8 @@ public class UnavailableChatService : IChatCompletionService
 #pragma warning restore OPENAI001
         bool enableContextualSearch = false,
         string? endUserId = null,
-        [System.Runtime.CompilerServices.EnumeratorCancellation] CancellationToken cancellationToken = default)
+        CancellationToken cancellationToken = default)
     {
-        yield return ("Chat service is unavailable in this environment.", responseId: null);
-        yield return (string.Empty, Guid.NewGuid().ToString("N"));
-        await Task.CompletedTask;
+        throw new ChatBackendUnavailableException("Chat service is unavailable in this environment.");
     }
 }