Add unit tests for example_util.convert_examples_to_text

[xxlest]

Summary

src/google/adk/examples/example_util.py:49 has a TODO from the original author:

# TODO(yaojie): Add unit tests for this function.
def convert_examples_to_text(examples: list[Example], model: Optional[str]) -> str:

This function handles few-shot example formatting for system instructions and currently has zero test coverage. The related build_example_si and _get_latest_message_from_user helpers are also untested.

Proposed test cases

• Basic text-only examples
• Function call + function response formatting
• Mixed content (text + function call)
• Empty examples list
• Gemini 2 vs non-Gemini 2 format branch (_FUNCTION_PREFIX vs _FUNCTION_CALL_PREFIX)
• build_example_si with list input vs BaseExampleProvider input
• _get_latest_message_from_user with empty session, non-user author, function responses

Happy to submit a PR for this.

[surajksharma07]

@xxlest the TODO at example_util.py:49 has been there since the file was written and these three functions have zero coverage, so a PR here is definitely welcome.

Target file would be tests/unittests/examples/test_example_util.py (the directory doesn't exist yet, so you'll need to add an __init__.py alongside it).

The proposed cases look solid; just make sure the Gemini-2 vs non-Gemini-2 prefix branch is covered for both function calls and function responses separately, as those follow different code paths at lines 79 and 85.

Run with pytest tests/unittests/examples/ from the repo root once the file is in place.

[surajksharma07]

@Jacksunwei PTAL.