Fix OpenAI response created timestamp (#16401)

## What this fixes

Closes #16400.

`get_data_openai()` currently returns `created: null` when callers do
not pass a timestamp, and it replaces explicit timestamp values with the
current time. This makes non-streaming OpenAI-compatible responses
inconsistent with the expected integer `created` timestamp field.

## Change

- Preserve explicit `created` values when provided.
- Default non-streaming responses to `int(time.time())` when `created`
is not provided.
- Add focused unit coverage for default timestamps, explicit timestamps,
and unchanged streaming chunk shape.

## Verification

- `./.venv/bin/python -m pytest
test/unit_test/api/utils/test_api_utils.py -q`
- `python3 -m py_compile api/utils/api_utils.py
test/unit_test/api/utils/test_api_utils.py`
- `uvx ruff check api/utils/api_utils.py
test/unit_test/api/utils/test_api_utils.py`

---------

Co-authored-by: Harsh Kashyap <harshkashyap@Harshs-MacBook-Pro.local>

api/utils/api_utils.py

@@ -444,7 +444,7 @@ def get_data_openai(id=None, created=None, model=None, prompt_tokens=0, completi
     return {
         "id": f"{id}",
         "object": object,
-        "created": int(time.time()) if created else None,
+        "created": created if created is not None else int(time.time()),
         "model": model,
         "param": param,
         "usage": {

test/unit_test/api/utils/test_api_utils.py

@@ -0,0 +1,48 @@
+#
+#  Copyright 2026 The InfiniFlow Authors. All Rights Reserved.
+#
+#  Licensed under the Apache License, Version 2.0 (the "License");
+#  you may not use this file except in compliance with the License.
+#  You may obtain a copy of the License at
+#
+#      http://www.apache.org/licenses/LICENSE-2.0
+#
+#  Unless required by applicable law or agreed to in writing, software
+#  distributed under the License is distributed on an "AS IS" BASIS,
+#  WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+#  See the License for the specific language governing permissions and
+#  limitations under the License.
+#
+
+from api.utils import api_utils
+
+
+def test_get_data_openai_defaults_created_to_current_timestamp(monkeypatch):
+    monkeypatch.setattr(api_utils.time, "time", lambda: 1234567890.9)
+
+    data = api_utils.get_data_openai(content="answer")
+
+    assert data["created"] == 1234567890
+
+
+def test_get_data_openai_preserves_explicit_created_value():
+    data = api_utils.get_data_openai(created=0, content="answer")
+
+    assert data["created"] == 0
+
+
+def test_get_data_openai_stream_response_shape_is_unchanged():
+    data = api_utils.get_data_openai(id="chatcmpl-test", model="test-model", content="chunk", finish_reason=None, stream=True)
+
+    assert data == {
+        "id": "chatcmpl-test",
+        "object": "chat.completion.chunk",
+        "model": "test-model",
+        "choices": [
+            {
+                "delta": {"content": "chunk"},
+                "finish_reason": None,
+                "index": 0,
+            }
+        ],
+    }