fix(auxiliary): propagate explicit_api_key to _try_anthropic()

_try_anthropic() lacked the explicit_api_key parameter added to
_try_openrouter() in #18768. When resolve_provider_client() is called
with provider="anthropic" and an explicit key (e.g. from a fallback_model
entry with api_key set), the key was silently ignored — _try_anthropic()
always fell back to resolve_anthropic_token(), so the fallback returned
None,None for users without a default Anthropic credential configured.

Fix: add explicit_api_key: str = None to _try_anthropic() and use
explicit_api_key or <pool/env fallback> in both the pool-present and
no-pool paths. Pass explicit_api_key=explicit_api_key at the call site
in resolve_provider_client(). Symmetric with the _try_openrouter() fix.
No behavior change when explicit_api_key is None.

agent/auxiliary_client.py

@@ -1529,7 +1529,7 @@ def _build_codex_client(model: str) -> Tuple[Optional[Any], Optional[str]]:
     return CodexAuxiliaryClient(real_client, model), model
 
 
-def _try_anthropic() -> Tuple[Optional[Any], Optional[str]]:
+def _try_anthropic(explicit_api_key: str = None) -> Tuple[Optional[Any], Optional[str]]:
     try:
         from agent.anthropic_adapter import build_anthropic_client, resolve_anthropic_token
     except ImportError:
@@ -1539,10 +1539,10 @@ def _try_anthropic() -> Tuple[Optional[Any], Optional[str]]:
     if pool_present:
         if entry is None:
             return None, None
-        token = _pool_runtime_api_key(entry)
+        token = explicit_api_key or _pool_runtime_api_key(entry)
     else:
         entry = None
-        token = resolve_anthropic_token()
+        token = explicit_api_key or resolve_anthropic_token()
     if not token:
         return None, None
 
@@ -2336,7 +2336,7 @@ def resolve_provider_client(
 
     if pconfig.auth_type == "api_key":
         if provider == "anthropic":
-            client, default_model = _try_anthropic()
+            client, default_model = _try_anthropic(explicit_api_key=explicit_api_key)
             if client is None:
                 logger.warning("resolve_provider_client: anthropic requested but no Anthropic credentials found")
                 return None, None