fix: validate audio sample rates

[fallintoplace]

Summary

• match rate only as a complete, case-insensitive MIME parameter
• reject non-positive source and destination sample rates before returning or resampling
• cover zero-rate inputs, bitrate parameters, and uppercase RATE parameters

Why

parse_sample_rate previously treated rate= as an arbitrary substring, so parameters such as bitrate=128000 could be interpreted as sample rates. It also accepted zero, which could reach a division by zero in resample_pcm16. Direct calls with a zero destination rate had the same problem.

Rejecting invalid rates explicitly avoids silently producing incorrectly pitched or timed audio.

Validation

• uv run pytest tests/unittests/evaluation/test_audio_utils.py -q
• uv run pre-commit run --files src/google/adk/evaluation/_audio_utils.py tests/unittests/evaluation/test_audio_utils.py