Make McpServerOptions.Filters and McpServerFilters sub-properties settable

- McpServerOptions.Filters is now get+set with null validation
- McpServerFilters.Message and McpServerFilters.Request are now get+set with null validation
- No other get-only/get-init properties found that should be get/set
  (IList<T> filter collections and internal McpServerImpl properties are correctly get-only)

Co-authored-by: stephentoub <2642209+stephentoub@users.noreply.github.com>

Author: Copilot

src/ModelContextProtocol.Core/Server/McpServerFilters.cs

@@ -12,12 +12,28 @@ namespace ModelContextProtocol.Server;
 public sealed class McpServerFilters
 {
     /// <summary>
-    /// Gets the filters for incoming and outgoing JSON-RPC messages.
+    /// Gets or sets the filters for incoming and outgoing JSON-RPC messages.
     /// </summary>
-    public McpMessageFilters Message { get; } = new();
+    public McpMessageFilters Message
+    {
+        get => field ??= new();
+        set
+        {
+            Throw.IfNull(value);
+            field = value;
+        }
+    }
 
     /// <summary>
-    /// Gets the filters for request-specific MCP handler pipelines.
+    /// Gets or sets the filters for request-specific MCP handler pipelines.
     /// </summary>
-    public McpRequestFilters Request { get; } = new();
+    public McpRequestFilters Request
+    {
+        get => field ??= new();
+        set
+        {
+            Throw.IfNull(value);
+            field = value;
+        }
+    }
 }

src/ModelContextProtocol.Core/Server/McpServerOptions.cs

@@ -108,14 +108,22 @@ public McpServerHandlers Handlers
     }
 
     /// <summary>
-    /// Gets the filter collections for MCP server handlers.
+    /// Gets or sets the filter collections for MCP server handlers.
     /// </summary>
     /// <remarks>
     /// This property provides access to filter collections that can be used to modify the behavior
     /// of various MCP server handlers. The first filter added is the outermost (first to execute),
     /// and each subsequent filter wraps closer to the handler.
     /// </remarks>
-    public McpServerFilters Filters { get; } = new();
+    public McpServerFilters Filters
+    {
+        get => field ??= new();
+        set
+        {
+            Throw.IfNull(value);
+            field = value;
+        }
+    }
 
     /// <summary>
     /// Gets or sets a collection of tools served by the server.