test(distributed): give each frontend its own data dir and one pinned secret

Session rows are keyed by an HMAC of the token under a secret generated
per instance into {DataPath}/.hmac_secret. The replicas shared that
secret only because they shared a working directory, and that directory
was the source tree. Give each frontend LOCALAI_DATA_PATH under its own
baseDir and pin LOCALAI_AUTH_HMAC_SECRET, so a session minted at one
replica resolves at every other one by construction.

Assisted-by: Claude Opus 5 [claude-code]
Signed-off-by: Ettore Di Giacinto <mudler@localai.io>
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