[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.

Inizia esaminando gli handler esistenti in src/Server/NotificationHandler e src/Server/RequestHandler, quindi confronta la loro copertura con Symfony MCP SDK e php-mcp's Dispatcher.php. L'issue identifica SetLogLevelRequest e CancelledNotification come ancora aperti, ma potrebbero rimanere altre lacune del protocollo. Il lavoro è completato quando sono stati concordati i sub-issue e i relativi handler per le richieste e le notifiche di protocollo mancanti.