fix(xai-oauth): pin inference base_url to x.ai origin (#28952)

XAI_BASE_URL / HERMES_XAI_BASE_URL let users repoint the OAuth-authenticated
inference endpoint, but the env override was an unguarded credential-leak
vector: a tampered .env or hostile shell init setting
XAI_BASE_URL=https://attacker.example/v1 would silently ship the SuperGrok
OAuth bearer to a third party on every request.

Add _xai_validate_inference_base_url() that pins the host to x.ai or a
*.x.ai subdomain and rejects non-HTTPS. On rejection, fall back to the
default with a warning rather than raise — a bad env var should not
deadlock auth, but should never leak the bearer either.

Apply at all three sites that read the env override for xai-oauth:
- hermes_cli/auth.py resolve_xai_oauth_runtime_credentials (main path)
- hermes_cli/auth.py _xai_oauth_loopback_login (initial login)
- agent/auxiliary_client.py _resolve_xai_oauth_for_aux (aux client)

E2E validated against four scenarios: attacker.example, lookalike
api.x.ai.evil.com, http:// downgrade on api.x.ai, and legit custom.x.ai
subdomain (which still resolves correctly).

Discovered while comparing against the opencode-grok-auth plugin
(github.com/ysnock404/opencode-grok-auth), which highlighted the same
guard on the OpenCode side.

agent/auxiliary_client.py

@@ -1307,7 +1307,10 @@ def _resolve_xai_oauth_for_aux() -> Optional[Tuple[str, str]]:
     with xAI Grok OAuth.
     """
     try:
-        from hermes_cli.auth import DEFAULT_XAI_OAUTH_BASE_URL
+        from hermes_cli.auth import (
+            DEFAULT_XAI_OAUTH_BASE_URL,
+            _xai_validate_inference_base_url,
+        )
 
         pool = load_pool("xai-oauth")
         if pool and pool.has_credentials():
@@ -1318,13 +1321,13 @@ def _resolve_xai_oauth_for_aux() -> Optional[Tuple[str, str]]:
                     or getattr(entry, "access_token", "")
                     or ""
                 ).strip()
-                base_url = str(
+                base_url = _xai_validate_inference_base_url(
                     os.getenv("HERMES_XAI_BASE_URL", "").strip().rstrip("/")
                     or os.getenv("XAI_BASE_URL", "").strip().rstrip("/")
-                    or getattr(entry, "runtime_base_url", None)
-                    or getattr(entry, "base_url", None)
-                    or DEFAULT_XAI_OAUTH_BASE_URL
-                ).strip().rstrip("/")
+                    or str(getattr(entry, "runtime_base_url", None) or "").strip().rstrip("/")
+                    or str(getattr(entry, "base_url", None) or "").strip().rstrip("/"),
+                    fallback=DEFAULT_XAI_OAUTH_BASE_URL,
+                )
                 if api_key and base_url:
                     return api_key, base_url
     except Exception as exc: