fix: cleanup from second agent review pass

- Remove dead MakeCacheOptions() method from ResponseIdValidationService
- Add comment explaining why user_question prompt is intentionally not
  XML-sanitized (C# generics like List<T> must not be corrupted)

Author: BenjaminMichaelis

EssentialCSharp.Chat.Shared/Services/AIChatService.cs

@@ -158,6 +158,10 @@ private async Task<string> EnrichPromptWithContext(string prompt, bool enableCon
 
         contextualInfo.AppendLine("</retrieved_context>");
         contextualInfo.AppendLine("<user_question>");
+        // The user's prompt is intentionally NOT passed through SanitizeForXmlContext:
+        // the user controls their own question (including C# generics like List<T>), and
+        // replacing '<'/'>' would corrupt code syntax in their query. The retrieved_context
+        // boundary above is protected; this tag is informational only.
         contextualInfo.AppendLine(prompt);
         contextualInfo.AppendLine("</user_question>");
 

EssentialCSharp.Web/Services/ResponseIdValidationService.cs

@@ -16,9 +16,6 @@ public sealed class ResponseIdValidationService(IMemoryCache cache)
 {
     private const int MaxTrackedIdsPerUser = 500;
 
-    private static MemoryCacheEntryOptions MakeCacheOptions() =>
-        new MemoryCacheEntryOptions().SetSlidingExpiration(TimeSpan.FromHours(2));
-
     /// <summary>
     /// Records a newly issued response ID as belonging to the specified user.
     /// </summary>