When agent_key is missing from auth state (expired, not yet minted,
or mint failed silently), the fallback chain fell through to
access_token — an OAuth bearer token for the Nous portal API, not
an inference credential. The Nous inference API returns 404 because
the OAuth token is not a valid inference key.
Remove the access_token fallback so an empty agent_key correctly
triggers resolve_nous_runtime_credentials() to mint a fresh key.
Closes#5562