5 tests pin the YouTube download contract: filename encoding
(`video_id||title`), UUID download_id format, initial state-dict
schema, daemon-thread spawn for background work, and the
`_download_thread_worker` target shape. Phase C will replace
the thread spawn with `engine.dispatch_download` — these tests
catch any drift in the per-download record shape that consumers
depend on.
Pure additive — no client code changes.
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