Fix MCP compatibility client shutdown deadlock

[JAORMX]

Summary

• release the compat-client mutex before waiting for go-sdk session shutdown
• clear only the session snapshot that was closed, preserving concurrent-close and initialization safety
• add a real go-sdk in-memory transport regression for notification-handler re-entry during shutdown

Root cause

Client.Close held c.mu while ClientSession.Close waited for active SDK notification handlers. A synchronous OnNotification callback re-entering ListPrompts then blocked on that mutex, deadlocking shutdown.

Validation

• go test -race ./mcpcompat/client -run TestClientClose_DoesNotHoldMutexWhileNotificationHandlerDrains -count=1
• task lint
• task test
• task

The new regression was also run against a temporary restoration of the affected implementation and failed because the callback could not re-enter the client; it passes with this change.