xAI's token endpoint returns HTTP 403 to the OAuth grant when the
account isn't on the allowlist for API access (e.g. standard
SuperGrok subscribers — see #26847). Treating it like a stale-token
400/401 made ``format_auth_error`` append "Run ``hermes model`` to
re-authenticate", which is misleading because re-login can't change
xAI's tier decision.
Split 403 off in both ``refresh_xai_oauth_pure`` and the loopback
login token exchange:
* New error code ``xai_oauth_tier_denied`` with ``relogin_required=False``
* Message explains the entitlement gate and points at the
``XAI_API_KEY`` + ``provider: xai`` fallback
* 400/401 still set ``relogin_required=True`` as before
* 5xx still set ``relogin_required=False`` as before
xxxigm
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