The Stop() method previously used a select/default pattern which was not
safe under concurrent calls — two goroutines could both pass the check
and attempt to close the same channel, causing a panic.
Replace with sync.Once to guarantee exactly-once close semantics,
matching the documented contract of being safe for concurrent use.
Review feedback: afjcjsbx
The SessionManager's background cleanup goroutine previously had no
shutdown mechanism. Each call to NewSessionManager() started a ticker
goroutine that ran indefinitely. In tests, where multiple
SessionManagers are created, this caused goroutine leaks.
This commit adds a Stop() method that cleanly shuts down the background
cleanup goroutine via a channel. Stop() is safe to call multiple times.
All existing tests now call t.Cleanup(sm.Stop) to ensure cleanup.
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