fix(codex): drop extra_headers for chatgpt.com backend

agent/transports/codex.py

@@ -218,22 +218,10 @@ class ResponsesApiTransport(ProviderTransport):
             kwargs.pop("timeout", None)
 
         if is_codex_backend:
-            prompt_cache_key = kwargs.get("prompt_cache_key")
-            cache_scope_id = str(prompt_cache_key or session_id or "").strip()
-            if cache_scope_id:
-                existing_extra_headers = kwargs.get("extra_headers")
-                merged_extra_headers: Dict[str, str] = {}
-                if isinstance(existing_extra_headers, dict):
-                    merged_extra_headers.update(
-                        {
-                            str(key): str(value)
-                            for key, value in existing_extra_headers.items()
-                            if key and value is not None
-                        }
-                    )
-                merged_extra_headers["session_id"] = cache_scope_id
-                merged_extra_headers["x-client-request-id"] = cache_scope_id
-                kwargs["extra_headers"] = merged_extra_headers
+            # chatgpt.com/backend-api/codex rejects body-level
+            # ``extra_headers`` with HTTP 400. Correlation/cache routing for
+            # this backend must not be sent through the Responses payload.
+            kwargs.pop("extra_headers", None)
 
         max_tokens = params.get("max_tokens")
         if max_tokens is not None and not is_codex_backend: