[Server] (Re-)Implement missing Notification & Request Handlers

For kicking off this SDK we used the Noticiation and Request handlers shipped with the Symfony MCP SDK and adopted schema component and registry.
But the MCP schema supports way more requests and notifications, that would need handlers as well. On php-mcp side this was the responsibility of the Dispatcher implementation.

Still open are:

• ListResourceTemplatesRequest
  #89
• ResourceSubscribeRequest
  #220
• ResourceUnsubscribeRequest
  #220
• SetLogLevelRequest
• CompletionCompleteRequest
  #97
• CancelledNotification

And potentially other requests and notification of the protocol.

Please announce yourself and create corresponding sub-issues before working on a specific handler - so we can avoid duplicate effort.

まず src/Server/NotificationHandler と src/Server/RequestHandler の既存のハンドラーを確認し、その後、Symfony MCP SDK および php-mcp's Dispatcher.php と対応範囲を比較します。この issue では SetLogLevelRequest と CancelledNotification が未対応として挙げられていますが、他のプロトコル上の不足が残っている可能性もあります。不足しているプロトコルリクエストと通知について、サブ issue と対応するハンドラーの合意が得られれば完了です。