Split configuration of request and message filters (#1308)

Co-authored-by: Copilot <175728472+Copilot@users.noreply.github.com>

src/ModelContextProtocol/IMcpMessageFilterBuilder.cs

@@ -0,0 +1,14 @@
+using ModelContextProtocol.Server;
+
+namespace Microsoft.Extensions.DependencyInjection;
+
+/// <summary>
+/// Provides a builder for configuring message-level MCP server filters.
+/// </summary>
+public interface IMcpMessageFilterBuilder
+{
+    /// <summary>
+    /// Gets the associated service collection.
+    /// </summary>
+    IServiceCollection Services { get; }
+}