fix(eval): handle unevaluated final response v2 results

[pragnyanramtha]

Summary

Fixes a small aggregation edge case in FinalResponseMatchV2Evaluator: when every per-invocation result is skipped or not evaluated, the evaluator currently divides by zero while computing the overall score.

Root Cause

aggregate_invocation_results() filters out results whose score is None or whose eval_status is NOT_EVALUATED, but it unconditionally computes:

overall_score = num_valid / num_evaluated

If all judge samples fail to produce a usable score, num_evaluated remains 0 and evaluation crashes instead of returning a not-evaluated aggregate result. Other ADK evaluators handle this condition by returning overall_score=None and overall_eval_status=NOT_EVALUATED.

Change

• Return an EvaluationResult with overall_score=None and overall_eval_status=NOT_EVALUATED when no FinalResponseMatchV2 invocation results are evaluable.
• Add a focused regression test for all-skipped/all-not-evaluated invocation results.

Validation

uv sync --extra test
uv run pytest tests/unittests/evaluation/test_final_response_match_v2.py

Result: 18 passed, 20 warnings.

Full unit suite was not run; this patch is limited to FinalResponseMatchV2 aggregation and its targeted unit test file.

[pragnyanramtha]

Refreshed this branch with current main in f61da061.

Validation rerun:

• uv run --extra test pytest tests/unittests/evaluation/test_final_response_match_v2.py -q (18 passed)
• uv run --extra dev pyink --check src/google/adk/evaluation/final_response_match_v2.py tests/unittests/evaluation/test_final_response_match_v2.py
• git diff --check

[pragnyanramtha]

Refreshed this branch with current main in f7a83e9b.

Validation rerun:

• uv run --extra test pytest tests/unittests/evaluation/test_final_response_match_v2.py -q (18 passed, 20 experimental warnings)
• uv run --extra dev pyink --check src/google/adk/evaluation/final_response_match_v2.py tests/unittests/evaluation/test_final_response_match_v2.py
• uv run --extra dev isort --check-only src/google/adk/evaluation/final_response_match_v2.py tests/unittests/evaluation/test_final_response_match_v2.py
• git diff --check