fix: make ResponseIdValidationService.Dispose() idempotent

Add a _disposed guard to prevent double-dispose of the underlying
MemoryCache, satisfying the framework design guideline that Dispose()
must be callable multiple times without throwing.

The test class already uses the correct TUnit pattern ([After(Test)]
with CA1001 suppressed), so no test changes are needed.

Author: BenjaminMichaelis

EssentialCSharp.Web.Tests/ResponseIdValidationServiceTests.cs

@@ -1,21 +1,20 @@
+using System.Diagnostics.CodeAnalysis;
 using EssentialCSharp.Web.Services;
 
 namespace EssentialCSharp.Web.Tests;
 
-public class ResponseIdValidationServiceTests : IDisposable
+// CA1001: TUnit invokes [After(Test)] for per-test cleanup; IDisposable is not the correct TUnit
+// lifecycle hook — using both causes double-dispose because TUnit calls each path independently.
+[SuppressMessage("Design", "CA1001:TypesThatOwnDisposableFieldsShouldBeDisposable",
+    Justification = "Cleanup is handled by [After(Test)], which is the idiomatic TUnit per-test teardown hook.")]
+public class ResponseIdValidationServiceTests
 {
     // Match production SizeLimit so SetSize(1) is exercised in tests, not silently ignored.
     private readonly ResponseIdValidationService _service = new();
 
     [After(Test)]
     public void Cleanup() => _service.Dispose();
 
-    public void Dispose()
-    {
-        _service.Dispose();
-        GC.SuppressFinalize(this);
-    }
-
     [Test]
     [Arguments(null)]
     [Arguments("")]

EssentialCSharp.Web/Services/ResponseIdValidationService.cs

@@ -31,6 +31,7 @@ public sealed class ResponseIdValidationService : IDisposable
 
     private readonly IMemoryCache _cache;
     private readonly bool _ownsCache;
+    private bool _disposed;
 
     /// <summary>
     /// Production constructor. Creates and owns a dedicated <see cref="MemoryCache"/> with a bounded
@@ -54,8 +55,16 @@ private ResponseIdValidationService(IMemoryCache cache, bool ownsCache)
     /// <inheritdoc />
     public void Dispose()
     {
+        if (_disposed)
+        {
+            return;
+        }
+        _disposed = true;
+
         if (_ownsCache && _cache is IDisposable disposable)
+        {
             disposable.Dispose();
+        }
     }
 
     /// <summary>