new_session() was popping the old cached session, releasing the lock,
calling get_or_create, then re-acquiring the lock to insert. A concurrent
caller could observe the empty-cache window and race-create its own
session, producing two divergent session objects for the same key.
_cache_lock is an RLock, so nested reacquisition inside get_or_create is
safe. Hold it across the whole pop/create/insert sequence.
Follow-up to #13510 (@hekaru-agent).
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