fix(llm): use the OpenAI Responses API only for native endpoints

The Responses API exists only on native OpenAI. When the openai provider is
pointed at a custom base_url (a proxy, gateway, or local server that speaks only
Chat Completions), keep the Responses API off so the call does not fail.

tests/test_openai_responses_base_url.py

@@ -0,0 +1,43 @@
+"""The Responses API only exists on native OpenAI; a custom base_url on the
+openai provider must fall back to Chat Completions (#1024)."""
+
+from __future__ import annotations
+
+import pytest
+
+from tradingagents.llm_clients.openai_client import (
+    OpenAIClient,
+    _is_native_openai_base_url,
+)
+
+
+@pytest.mark.unit
+class NativeBaseUrlTests:
+    def test_unset_is_native(self):
+        assert _is_native_openai_base_url(None) is True
+        assert _is_native_openai_base_url("") is True
+
+    def test_openai_hosts_are_native(self):
+        assert _is_native_openai_base_url("https://api.openai.com/v1") is True
+        assert _is_native_openai_base_url("api.openai.com/v1") is True
+
+    def test_custom_endpoints_are_not_native(self):
+        assert _is_native_openai_base_url("http://localhost:1234/v1") is False
+        assert _is_native_openai_base_url("https://my-gateway.example.com/v1") is False
+        assert _is_native_openai_base_url("https://api.openai.com.evil.com/v1") is False
+
+
+@pytest.mark.unit
+class ResponsesApiSelectionTests:
+    def test_native_openai_enables_responses_api(self, monkeypatch):
+        monkeypatch.setenv("OPENAI_API_KEY", "sk-test")
+        llm = OpenAIClient("gpt-5.5", provider="openai").get_llm()
+        assert getattr(llm, "use_responses_api", False) is True
+
+    def test_custom_base_url_disables_responses_api(self, monkeypatch):
+        monkeypatch.setenv("OPENAI_API_KEY", "sk-test")
+        llm = OpenAIClient(
+            "gpt-5.5", base_url="http://localhost:1234/v1", provider="openai"
+        ).get_llm()
+        # use_responses_api should be absent/False so the client speaks Chat Completions.
+        assert getattr(llm, "use_responses_api", False) is False