fix: handle legacy Lyria output accessors

[he-yufeng]

Summary

• detect legacy Lyria responses when Vertex returns a full model resource path
• rewrite legacy outputs into modern steps for that path
• make output_image, output_audio, and output_video tolerate missing steps, matching the existing output_text behavior

Fixes #2534.

Verification

$env:PYTHONPATH='.'; python -m pytest google\genai\tests\interactions\test_legacy_lyria.py -q
python -m py_compile google\genai\_interactions\_legacy_lyria.py google\genai\_interactions\types\interaction.py google\genai\tests\interactions\test_legacy_lyria.py
python -m ruff check --select E9,F63,F7,F82 google\genai\_interactions\_legacy_lyria.py google\genai\_interactions\types\interaction.py google\genai\tests\interactions\test_legacy_lyria.py
python -m ruff format --diff google\genai\tests\interactions\test_legacy_lyria.py
git diff --check