test: add unit tests for _detect_docstring_style

[xiaweiwei67-stack]

Summary

_detect_docstring_style in src/agents/function_schema.py approximates griffe's docstring-style auto-detection (used when building parameter descriptions for function tools), but it had no direct unit tests. This adds coverage for:

• detection of each style (Sphinx field markers, NumPy dashed-underline headers, Google section headers),
• the google fallback for docstrings with no recognizable markers and for an empty docstring,
• the documented tie-break priority (sphinx > numpy > google) when scores are equal.

Tests only - no source or behavior changes.

Test plan

• uv run pytest tests/test_function_schema.py -k detect_docstring_style -> 8 passed
• ruff check and ruff format --check -> clean on the changed file
• mypy -> clean on the changed file

Issue number

N/A

Checks

• I've added new tests (if relevant)
• I've added/updated the relevant documentation
• I've run make lint and make format
• I've made sure tests pass

[seratch]

Thanks for sharing this. However, we don't need this test addition.