[Agent][Yandex Ingestion] Harden metadata-only Yandex Music ingestion

[karnaksp]

Objective

Keep real-account ingestion token-safe, idempotent, and resilient to unofficial API edge cases.

Acceptance

• make preflight returns safe account counts without writing raw data.
• make ingest writes normalized JSONL/Parquet metadata only.
• make raw-contract passes.
• Manifest diagnostics explain skipped, duplicate, and fallback rows.

Notes

The adapter uses an unofficial Yandex Music client; document API risk and never store audio.