Refactor handler context to be transport- and handler-type-aware

[maxisbey]

The current RequestContext / ServerRequestContext design has a few rough edges that warrant a dedicated refactor:

1. Request vs notification context — Request handlers have request_id and meta, notification handlers do not. Currently request_id is optional (RequestId | None) on a single shared type, which loses type safety. Ideally these would be distinct types so request handlers get a context where request_id is always present, and notification handlers get one where it does not exist.

2. Transport-specific context — The request_context field on ServerMessageMetadata is typed as Any because the server layer is transport-agnostic (e.g. it is a Starlette Request for HTTP transports, None for stdio). A better design would let transports provide their own typed context that handlers can access without casting.

These two concerns are related — both point toward making the context type more precise depending on how and where a handler is invoked.

_{AI Disclaimer}