When xAI's auth backend fails to redirect (e.g. the German "We couldn't reach
your app" fallback shown in #27385), users sometimes navigate manually to the
bare loopback callback URL — `http://127.0.0.1:<port>/callback` with no query
string. The handler used to return 200 "xAI authorization received" for any
GET that hit the expected path, because `parse_qs("")` yields no `code` and no
`error`, leaving `result` untouched while the success page was still served.
The CLI's wait loop, of course, still saw no code and timed out with
`AuthError: xAI authorization timed out waiting for the local callback.`
The user is left looking at a browser tab that claims success and a terminal
that says failure — exactly the contradiction in #27385.
This change makes the empty-callback case return 400 with an explicit
"not received" page and a hint to retry `hermes auth add xai-oauth`. The
wait-loop semantics are unchanged: `result["code"]` and `result["error"]`
both stay None, so the CLI still raises a real timeout rather than treating
the bare hit as a successful callback.
Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>
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