fix: per-call MemoryCacheEntryOptions and add TokenLimitExceededException docs

- ResponseIdValidationService: create a fresh MemoryCacheEntryOptions per
  RecordResponseId call instead of sharing a static instance. The shared
  instance was safe today (no mutable lists populated) but is a latent hazard
  if ExpirationTokens or PostEvictionCallbacks are ever added in future —
  IMemoryCache reads those lists at Set time without defensive copying
- TokenLimitExceededException: add XML doc warning that catch-by-base-class
  (InvalidOperationException) will silently capture this type

Author: BenjaminMichaelis

EssentialCSharp.Web/Services/ResponseIdValidationService.cs

@@ -20,9 +20,6 @@ namespace EssentialCSharp.Web.Services;
 /// </remarks>
 public sealed class ResponseIdValidationService(IMemoryCache cache)
 {
-    private static readonly MemoryCacheEntryOptions _EntryOptions =
-        new MemoryCacheEntryOptions().SetSlidingExpiration(TimeSpan.FromHours(2));
-
     /// <summary>
     /// Records a newly issued response ID as belonging to the specified user.
     /// </summary>
@@ -33,7 +30,11 @@ public void RecordResponseId(string? userId, string? responseId)
             return;
         }
 
-        cache.Set(ResponseKey(responseId), userId, _EntryOptions);
+        // Create a fresh options instance per call — MemoryCacheEntryOptions has mutable
+        // list properties (ExpirationTokens, PostEvictionCallbacks) and sharing a static
+        // instance would cause future additions to affect all entries.
+        var entryOptions = new MemoryCacheEntryOptions().SetSlidingExpiration(TimeSpan.FromHours(2));
+        cache.Set(ResponseKey(responseId), userId, entryOptions);
     }
 
     /// <summary>

EssentialCSharp.Web/Services/TokenLimitExceededException.cs

@@ -5,5 +5,10 @@ namespace EssentialCSharp.Web.Services;
 /// Using a dedicated type (rather than base <see cref="InvalidOperationException"/>) lets callers
 /// discriminate this expected business-rule failure from unexpected EF Core / runtime failures.
 /// </summary>
+/// <remarks>
+/// This type inherits <see cref="InvalidOperationException"/> for convenience. Be aware that any
+/// <c>catch (InvalidOperationException)</c> guard added in the call chain will silently capture
+/// this exception. Prefer catching <see cref="TokenLimitExceededException"/> directly.
+/// </remarks>
 public sealed class TokenLimitExceededException(int limit)
     : InvalidOperationException($"You have reached the maximum of {limit} active MCP tokens.");