When a token refresh fails with a fatal OAuth2 error (invalid_grant,
invalid_client), the session is now cleared from the session store
and the cookie is removed, forcing re-authentication.
Previously, fatal refresh errors were logged but the stale session
continued to be served, leaving users logged in indefinitely after
their session was revoked at the provider level.
Transient errors (network timeouts, server errors) continue to
preserve the existing session as before.
Fixes#1945
c2f27999bd