Seal McpClientHandlers and add setter to McpServerOptions.Handlers for consistency

- McpClientHandlers is now sealed, matching McpServerHandlers
- McpServerOptions.Handlers now has get+set with null validation, matching McpClientOptions.Handlers

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

Author: Copilot

src/ModelContextProtocol.Core/Client/McpClientHandlers.cs

@@ -22,7 +22,7 @@ namespace ModelContextProtocol.Client;
 /// is done based on an ordinal, case-sensitive string comparison.
 /// </para>
 /// </remarks>
-public class McpClientHandlers
+public sealed class McpClientHandlers
 {
     /// <summary>Gets or sets notification handlers to register with the client.</summary>
     /// <remarks>

src/ModelContextProtocol.Core/Server/McpServerOptions.cs

@@ -97,7 +97,15 @@ public sealed class McpServerOptions
     /// <summary>
     /// Gets or sets the container of handlers used by the server for processing protocol messages.
     /// </summary>
-    public McpServerHandlers Handlers { get; } = new();
+    public McpServerHandlers Handlers
+    {
+        get => field ??= new();
+        set
+        {
+            Throw.IfNull(value);
+            field = value;
+        }
+    }
 
     /// <summary>
     /// Gets the filter collections for MCP server handlers.