Expose client version asset availability

[mattiapearl]

Summary

• Add a helper for per-client-version payload coverage.
• Expose GET /v2/client-version-availability.
• Include items, generic_data, images, and nearest_images_version fields.

Tests

• PYTHONPATH=. python -m pytest tests/test_client_version_availability.py -q
• python -m ruff format deadlock_assets_api tests/test_client_version_availability.py
• python -m ruff check deadlock_assets_api tests/test_client_version_availability.py
• PYTHONPATH=. python -m compileall -q deadlock_assets_api tests/test_client_version_availability.py