test: update empty output test for finish_reason-aware handling

The existing test used finish_reason="stop" which now returns gracefully.
Change it to finish_reason="length" to keep testing the error path, and
add a new test verifying that finish_reason="stop" with empty content
returns a valid response (GPT-5 behavior).

Author: he-yufeng

tests/test_openai_source.py

@@ -1142,6 +1142,7 @@ async def fake_create(**kwargs):
 
 @pytest.mark.asyncio
 async def test_parse_openai_completion_raises_empty_model_output_error():
+    """Empty output with non-stop finish_reason should raise."""
     provider = _make_provider()
     try:
         completion = ChatCompletion.model_validate(
@@ -1159,7 +1160,7 @@ async def test_parse_openai_completion_raises_empty_model_output_error():
                             "refusal": None,
                             "tool_calls": None,
                         },
-                        "finish_reason": "stop",
+                        "finish_reason": "length",
                     }
                 ],
                 "usage": {
@@ -1176,6 +1177,44 @@ async def test_parse_openai_completion_raises_empty_model_output_error():
         await provider.terminate()
 
 
+@pytest.mark.asyncio
+async def test_parse_openai_completion_empty_content_finish_stop_no_error():
+    """Empty output with finish_reason=stop should return gracefully (GPT-5 etc.)."""
+    provider = _make_provider()
+    try:
+        completion = ChatCompletion.model_validate(
+            {
+                "id": "chatcmpl-empty-stop",
+                "object": "chat.completion",
+                "created": 0,
+                "model": "gpt-5.4",
+                "choices": [
+                    {
+                        "index": 0,
+                        "message": {
+                            "role": "assistant",
+                            "content": None,
+                            "refusal": None,
+                            "tool_calls": None,
+                        },
+                        "finish_reason": "stop",
+                    }
+                ],
+                "usage": {
+                    "prompt_tokens": 13,
+                    "completion_tokens": 18,
+                    "total_tokens": 31,
+                },
+            }
+        )
+
+        resp = await provider._parse_openai_completion(completion, tools=None)
+        assert resp.role == "assistant"
+        assert resp.result_chain is not None
+    finally:
+        await provider.terminate()
+
+
 @pytest.mark.asyncio
 async def test_query_stream_extracts_usage_from_empty_choices_chunk(monkeypatch):
     provider = _make_provider()