## Summary
- Treat explicit null collection fields in Bilibili VOD playback
responses as empty collections.
- Cover DASH streams, backup URLs, Dolby audio, supported formats, DURL
responses, subtitles, video pages, UGC collections, and PGC episodes.
- Resolve Bilibili DASH resources from versioned playback metadata and
provide every equivalent CDN URL to the existing candidate executor.
- Prefer the URL selected by the player, then retry primary and backup
CDNs before committing the downstream response.
## Root cause
Bilibili occasionally emits explicit null for fields normally
represented as JSON arrays, which caused playback/start deserialization
errors. Proxy DASH resource requests also used a single CDN even though
the provider response included backup URLs.
## Validation
- cargo fmt --all -- --check
- cargo check for synctv-api-common, synctv-api-http, and
synctv-api-grpc
- synctv-media-providers Bilibili tests: 91 passed
- synctv-core Bilibili tests: 17 passed, 2 ignored
- candidate executor tests: 2 passed
- Real playback/start, proxy MPD, Range request, CDN fault injection,
dev build, and dev smoke checks
zijiren
zijiren233