Spec: Logging: Per-request log level

The server MUST NOT emit notifications/message for a request that does not include this field. […] notifications/message is request-scoped: the server MUST NOT deliver it on a subscriptions/listen stream or on any stream other than the one carrying the response to the request that set the log level.

Current behavior

McpAsyncNotifier.notify(method, params) sends any notification without checks. A handler can send notifications/message without a logLevel (or below the requested level), or notification types that don't belong to the request.

Proposal

Let users inject a notifier factory, so the policy is the user's choice and the framework doesn't hard-code one. A factory could:

• drop or reject notifications by method
• gate log messages on logLevel and the requested threshold
• check extension-specific rules

The default stays permissive – the factory is the place to add stricter rules.