C# Re-implementation (#5)

Re-implementation of C# interceptors SDK with clearer focus on
server-side / gateway interceptors.
Fixes a lot of issues with the existing implementation.
Still more boilerplate than I'm happy with, but this is better than what
was there. Will refine further in later PRs.

csharp/sdk/samples/AvatarMoodInterceptorSample/AvatarInterceptors.cs

@@ -0,0 +1,52 @@
+using System.Text.Json.Nodes;
+using Microsoft.Extensions.AI;
+using ModelContextProtocol.Interceptors.Protocol;
+using ModelContextProtocol.Interceptors.Server;
+
+namespace AvatarMoodInterceptorSample;
+
+/// <summary>
+/// A sink interceptor that classifies conversational mood from each
+/// <c>llm/completion</c> response using a smaller, secondary model (Haiku) and
+/// updates a console avatar. Demonstrates that sink interceptors can do
+/// real work — driving downstream state — without gating the main conversation.
+/// </summary>
+[McpServerInterceptorType]
+public sealed class AvatarInterceptors
+{
+    private readonly IChatClient _haiku;
+    private readonly string _haikuModelId;
+    private readonly AvatarState _state;
+
+    public AvatarInterceptors(IChatClient haiku, string haikuModelId, AvatarState state)
+    {
+        _haiku = haiku;
+        _haikuModelId = haikuModelId;
+        _state = state;
+    }
+
+    [McpServerInterceptor(
+        Name = "avatar-mood",
+        Description = "Classifies conversation mood via a secondary model and updates the avatar state.",
+        Type = InterceptorType.Sink,
+        Events = [InterceptionEvents.LlmCompletion],
+        Phase = InterceptorPhase.Response)]
+    public SinkInterceptorResult OnLlmCompletion(
+        JsonNode payload,
+        string @event,
+        InterceptorPhase phase,
+        InvokeInterceptorContext? context,
+        CancellationToken cancellationToken)
+    {
+        // Sink contract: parallel, fire-and-forget, failures swallowed.
+        // We return immediately so the main conversation never waits on Haiku.
+        _ = Task.Run(async () =>
+        {
+            var verdict = await MoodClassifier.ClassifyAsync(_haiku, payload, cancellationToken);
+            _state.Update(verdict.Label, verdict.Confidence, _haikuModelId);
+            AvatarRenderer.Render(_state);
+        }, cancellationToken);
+
+        return new SinkInterceptorResult { Recorded = true };
+    }
+}