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/InterceptorServerSample/SampleInterceptors.cs

@@ -0,0 +1,95 @@
+using System.Text.Json.Nodes;
+using ModelContextProtocol.Interceptors.Protocol;
+using ModelContextProtocol.Interceptors.Server;
+
+/// <summary>
+/// Sample interceptors demonstrating validation, mutation, and sink types.
+/// </summary>
+[McpServerInterceptorType]
+public class SampleInterceptors
+{
+    /// <summary>
+    /// Validates that tool call arguments don't contain PII patterns.
+    /// </summary>
+    [McpServerInterceptor(
+        Name = "pii-validator",
+        Description = "Checks tool call arguments for PII patterns",
+        Type = InterceptorType.Validation,
+        Events = [InterceptionEvents.ToolsCall],
+        Phase = InterceptorPhase.Request)]
+    public static ValidationInterceptorResult ValidatePii(JsonNode payload)
+    {
+        var json = payload.ToJsonString();
+
+        // Simple pattern check for demonstration
+        if (json.Contains("ssn", StringComparison.OrdinalIgnoreCase) ||
+            json.Contains("social security", StringComparison.OrdinalIgnoreCase))
+        {
+            return ValidationInterceptorResult.Failure(
+                new ValidationMessage
+                {
+                    Path = "$.arguments",
+                    Message = "Payload may contain Social Security Number data",
+                    Severity = ValidationSeverity.Error,
+                });
+        }
+
+        return ValidationInterceptorResult.Success();
+    }
+
+    /// <summary>
+    /// Redacts email addresses from tool call payloads.
+    /// </summary>
+    [McpServerInterceptor(
+        Name = "email-redactor",
+        Description = "Redacts email addresses from payloads",
+        Type = InterceptorType.Mutation,
+        Events = [InterceptionEvents.ToolsCall],
+        Phase = InterceptorPhase.Request,
+        PriorityHint = -1000)]
+    public static MutationInterceptorResult RedactEmails(JsonNode payload)
+    {
+        var json = payload.ToJsonString();
+        var redacted = System.Text.RegularExpressions.Regex.Replace(
+            json,
+            @"[a-zA-Z0-9._%+-]+@[a-zA-Z0-9.-]+\.[a-zA-Z]{2,}",
+            "[EMAIL_REDACTED]");
+
+        if (redacted != json)
+        {
+            return new MutationInterceptorResult
+            {
+                Modified = true,
+                Payload = JsonNode.Parse(redacted),
+            };
+        }
+
+        return new MutationInterceptorResult { Modified = false, Payload = payload };
+    }
+
+    /// <summary>
+    /// Logs all intercepted events.
+    /// </summary>
+    [McpServerInterceptor(
+        Name = "request-logger",
+        Description = "Logs all requests",
+        Type = InterceptorType.Sink,
+        Events = [InterceptionEvents.All])]
+    public static SinkInterceptorResult LogRequest(
+        JsonNode payload,
+        string @event,
+        InterceptorPhase phase,
+        InvokeInterceptorContext? context)
+    {
+        Console.Error.WriteLine($"[interceptor] event={@event} phase={phase} traceId={context?.TraceId ?? "none"} payloadSize={payload.ToJsonString().Length}");
+
+        return new SinkInterceptorResult
+        {
+            Recorded = true,
+            Metrics = new Dictionary<string, double>
+            {
+                ["payloadBytes"] = payload.ToJsonString().Length,
+            },
+        };
+    }
+}