Tests:
* ``test_refresh_xai_oauth_pure_403_marked_tier_denied_not_relogin`` —
refresh-403 raises ``xai_oauth_tier_denied`` with
``relogin_required=False`` and the API-key fallback hint in body.
* ``test_format_auth_error_tier_denied_does_not_suggest_relogin`` —
the renderer does not append "Run ``hermes model``" for the new
code.
* ``test_recover_with_credential_pool_skips_refresh_on_bare_403_for_xai_oauth`` —
bare ``{"reason":"forbidden","message":"Forbidden"}`` body (which
does not match the existing keyword heuristic) still short-circuits
``try_refresh_current`` on xai-oauth.
Docs:
* Drop the "(any active tier)" claim from the xai-grok-oauth guide,
add a top-of-page warning callout, and a Troubleshooting section
for the 403-after-login case pointing at ``XAI_API_KEY`` +
``provider: xai`` as the documented fallback.
xxxigm
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