### What problem does this PR solve?
Closes#8175.
The Agent OpenAI-compatible streaming path uses `get_data_openai(...,
stream=True)`, but that helper currently returns a minimal chunk shape.
The main OpenAI-compatible chat endpoint already includes chunk metadata
such as `created`, `system_fingerprint`, `usage`, `logprobs`, and
assistant role/tool placeholders.
This PR aligns the Agent stream helper with that existing
OpenAI-compatible chunk shape while keeping the current `delta.content`
behavior and existing reference injection path intact.
### Type of change
- [x] Bug Fix (non-breaking change which fixes an issue)
- [ ] New Feature (non-breaking change which adds functionality)
- [ ] Documentation Update
- [ ] Refactoring
- [ ] Performance Improvement
- [ ] Other (please describe):
### 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>
## 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>
