mcp: reject resource subscriptions during close

[jstar0]

Fixes #1171.

This change makes SEP-2575 resource subscription startup follow the same close semantics as other client sends:

• ClientSession.Close marks the session as closing before cancelling existing listen streams.
• ClientSession.Subscribe rejects calls after closing begins.
• failed subscriptions/listen startup rolls back the resourceSubs entry it just registered.
• subscriptions/listen now reports immediate jsonrpc2 close errors back to the caller instead of silently discarding them.

Tests:

• go test ./mcp -run 'TestResourceSubscriptions_(SubscribeAfterCloseFails|SubscribeConcurrentCloseFails|Subscribe_Idempotent|InMemory|MultipleURIs)$'
• go test ./internal/jsonrpc2 ./mcp -run 'TestSubscriptionsListen|TestResourceSubscriptions'
• go test -race ./mcp -run 'TestResourceSubscriptions_SubscribeConcurrentCloseFails$'
• go test ./... (fails only TestSSELocalhostProtection/0.0.0.0_via_localhost_rejects_evil.com and TestStreamableLocalhostProtection/0.0.0.0_via_localhost_rejects_evil.com; both got HTTP 200 instead of the expected 403, and the same failures reproduce when those tests are run directly)