fix(xai-proxy): handle 429 rate-limit responses in proxy retry path

get_retry_credential only triggered on 401; a 429 Too Many Requests from
xAI was silently streamed back with no key rotation or back-off signal.

- server.py: widen retry gate from == 401 to in {401, 429}
- xai.py: on 429, skip token refresh and call mark_exhausted_and_rotate
  to stamp the 1-hour cooldown on the rate-limited key and return the
  next available credential. Returns None if pool is exhausted.

hermes_cli/proxy/server.py

@@ -206,7 +206,7 @@ def create_app(adapter: UpstreamAdapter) -> "web.Application":
             return session_or_response
         session = session_or_response
 
-        if upstream_resp.status == 401:
+        if upstream_resp.status in {401, 429}:
             try:
                 retry_cred = adapter.get_retry_credential(
                     failed_credential=cred,