fix(gateway): honor key_env in auth-failure fallback resolution

gateway/run.py

@@ -1004,10 +1004,17 @@ def _try_resolve_fallback_provider() -> dict | None:
             if not isinstance(entry, dict):
                 continue
             try:
+                explicit_api_key = entry.get("api_key")
+                if not explicit_api_key:
+                    key_env = str(
+                        entry.get("key_env") or entry.get("api_key_env") or ""
+                    ).strip()
+                    if key_env:
+                        explicit_api_key = os.getenv(key_env, "").strip() or None
                 runtime = resolve_runtime_provider(
                     requested=entry.get("provider"),
                     explicit_base_url=entry.get("base_url"),
-                    explicit_api_key=entry.get("api_key"),
+                    explicit_api_key=explicit_api_key,
                 )
                 logger.info(
                     "Fallback provider resolved: %s model=%s",