mcp: reject resource subscriptions during close

[jstar0]

Fixes #1171.

This change prevents new SEP-2575 resource subscriptions from racing ClientSession.Close:

• Close marks the session as closing before cancelling active listen streams.
• Subscribe checks the close state before and after registering the listen, and clears its local entry if startup fails or close wins the race.
• Connect treats server rejection of the optional list-changed subscriptions/listen as non-fatal; an explicit resource Subscribe still returns the rejection.

Tests:

• go test ./... — passed.
• go test -race ./mcp -run 'TestResourceSubscriptions_SubscribeConcurrentCloseFails$' — passed.
• go test ./mcp -run 'TestStreamableClient_(StatelessSubscriptionsListen404|ResourceSubscribeListen404ReturnsError)|TestResourceSubscriptions' -count=1 — passed.

[jstar0]

Rebased the PR onto current main and updated head 10ab202 after preserving the close-race fix against the latest client lifecycle changes. One compatibility boundary is explicit now: rejection of Connect's optional list-changed listen leaves the session usable, while an explicit resource Subscribe still returns the rejection and clears its registration.

Verification on the current head: go test ./... passed; the focused listen/resource-subscription tests and go test -race ./mcp -run TestResourceSubscriptions_SubscribeConcurrentCloseFails passed; all 9 hosted checks, including both conformance jobs and the race job, passed.

Could you review this current head when convenient?