fix(auxiliary): propagate explicit_api_key to _try_openrouter()

When resolve_provider_client() passes explicit_api_key for OpenRouter auxiliary
tasks, _try_openrouter() now accepts and honors this parameter instead of
silently ignoring it and falling back to OPENROUTER_API_KEY env var.

Root cause: _try_openrouter() had no explicit_api_key parameter, so even
when callers wanted to pass a runtime credential pool key, it could not be used.

Fix:
- Add explicit_api_key: str = None parameter to _try_openrouter()
- Prioritize explicit_api_key over pool key and env var
- Update resolve_provider_client() call site to pass explicit_api_key

Regression coverage:
- Test that explicit_api_key is passed to OpenAI client when provided
- Test that fallback to OPENROUTER_API_KEY still works when explicit_api_key is None

Closes #18338

agent/auxiliary_client.py

@@ -1149,10 +1149,10 @@ def _resolve_api_key_provider() -> Tuple[Optional[OpenAI], Optional[str]]:
 
 
 
-def _try_openrouter() -> Tuple[Optional[OpenAI], Optional[str]]:
+def _try_openrouter(explicit_api_key: str = None) -> Tuple[Optional[OpenAI], Optional[str]]:
     pool_present, entry = _select_pool_entry("openrouter")
     if pool_present:
-        or_key = _pool_runtime_api_key(entry)
+        or_key = explicit_api_key or _pool_runtime_api_key(entry)
         if not or_key:
             return None, None
         base_url = _pool_runtime_base_url(entry, OPENROUTER_BASE_URL) or OPENROUTER_BASE_URL
@@ -1160,7 +1160,7 @@ def _try_openrouter() -> Tuple[Optional[OpenAI], Optional[str]]:
         return OpenAI(api_key=or_key, base_url=base_url,
                        default_headers=_OR_HEADERS), _OPENROUTER_MODEL
 
-    or_key = os.getenv("OPENROUTER_API_KEY")
+    or_key = explicit_api_key or os.getenv("OPENROUTER_API_KEY")
     if not or_key:
         return None, None
     logger.debug("Auxiliary client: OpenRouter")
@@ -2053,9 +2053,9 @@ def resolve_provider_client(
         return (_to_async_client(client, final_model, is_vision=is_vision) if async_mode
                 else (client, final_model))
 
-    # ── OpenRouter ───────────────────────────────────────────────────
+    # ── OpenRouter ───────────────────────────────────────────
     if provider == "openrouter":
-        client, default = _try_openrouter()
+        client, default = _try_openrouter(explicit_api_key=explicit_api_key)
         if client is None:
             logger.warning(
                 "resolve_provider_client: openrouter requested but %s",