Adds testEmailPasswordSessionNotCorruptedByConcurrentRequests which
reproduces the cross-worker Redis cache race that caused 401s after
login. The test fires a login request, waits for it to reach the cache
purge point, then injects concurrent GET /v1/account requests that
re-cache a stale user document. Verifies the new session is immediately
usable.
Fails against the old ordering (purge before create), passes with the
fix (create before purge).
Chirag Aggarwal
6dba407aed