mcp: reject Subscribe on a closed session and roll back failed registrations

[GerardGao]

What

ClientSession.Subscribe (SEP-2575 path) starts a subscriptions/listen stream
without awaiting the send result, so on a session that is closing — or when the
listen send fails for any other reason — it can return nil even though the
stream never started. The local resourceSubs map also keeps an entry whose
cancel func is then never called, leaking the registration and misreporting the
URI as subscribed. Fixes #1171.

How

• Add a closed flag to ClientSession, set at the top of Close.
• Subscribe checks the flag under resourceSubsMu before registering, so a
  call after Close has started returns ErrConnectionClosed and registers
  nothing. Because the check and the registration share the mutex with the
  Close cleanup sweep, a racing registration is either swept by Close or
  refused outright — no interleaving leaks.
• If subscriptionsListen reports an error, the registration is rolled back
  (entry removed and cancel func called), so a later Subscribe retries the
  listen instead of treating the URI as already subscribed.

Unsubscribe and idempotent double-Subscribe behavior are unchanged.

Tests

• TestResourceSubscriptions_SubscribeClosedSession: Subscribe after Close
  returns ErrConnectionClosed and leaves no resourceSubs entry.
• TestResourceSubscriptions_SubscribeListenFailureRollsBack: a failed listen
  send (injected via AddSendingMiddleware) rolls back the registration and a
  retry re-attempts the listen.
• TestResourceSubscriptions_SubscribeCloseRace: 50 rounds of racing Close
  and Subscribe asserting that a failed Subscribe never leaves an entry.

go test -race ./mcp/ and go test ./... pass; gofmt, go vet, and
staticcheck are clean.