fix(client): handle is_closed as method in OpenAI SDK

The openai SDK's SyncAPIClient.is_closed is a method, not a property.
getattr(client, 'is_closed', False) returned the bound method object,
which is always truthy — causing _is_openai_client_closed() to report
all clients as closed and triggering unnecessary client recreation
(~100-200ms TCP+TLS overhead per API call).

Fix: check if is_closed is callable and call it, otherwise treat as bool.

Fixes #4377
Co-authored-by: Bartok9 <Bartok9@users.noreply.github.com>

run_agent.py

@@ -3486,14 +3486,33 @@ class AIAgent:
 
     @staticmethod
     def _is_openai_client_closed(client: Any) -> bool:
+        """Check if an OpenAI client is closed.
+
+        Handles both property and method forms of is_closed:
+        - httpx.Client.is_closed is a bool property
+        - openai.OpenAI.is_closed is a method returning bool
+
+        Prior bug: getattr(client, "is_closed", False) returned the bound method,
+        which is always truthy, causing unnecessary client recreation on every call.
+        """
         from unittest.mock import Mock
 
         if isinstance(client, Mock):
             return False
-        if bool(getattr(client, "is_closed", False)):
-            return True
+
+        is_closed_attr = getattr(client, "is_closed", None)
+        if is_closed_attr is not None:
+            # Handle method (openai SDK) vs property (httpx)
+            if callable(is_closed_attr):
+                if is_closed_attr():
+                    return True
+            elif bool(is_closed_attr):
+                return True
+
         http_client = getattr(client, "_client", None)
-        return bool(getattr(http_client, "is_closed", False))
+        if http_client is not None:
+            return bool(getattr(http_client, "is_closed", False))
+        return False
 
     def _create_openai_client(self, client_kwargs: dict, *, reason: str, shared: bool) -> Any:
         if self.provider == "copilot-acp" or str(client_kwargs.get("base_url", "")).startswith("acp://copilot"):