Validate live responses against models in CI (drift detection)

[sigma67]

Part of #307. Base branch: ytmusicapi-2. Blocked until the Phase 2 issues have landed.

The original motivation for #307: notice sooner when YouTube Music changes something, instead of finding out from a user's bug report.

Scope

Add validation of live responses against the models in CI. Today's tests mostly assert on result counts, which catches a section disappearing but not a field changing type, being renamed or quietly vanishing.

Approach to settle in this issue: the models ship with extra="allow" for consumer safety (see #980), so the drift check needs to be stricter than normal runtime behaviour — for example validating with extra="forbid" in tests so a newly appearing YTM field surfaces as a failure rather than being silently absorbed.

Watch out for

• This must fail loudly but not flakily. YTM A/B tests mean some variation is normal and expected — see the a/b label and fix: skip unexpected renderer types in parse_content_list #970.
• Consider running it on a schedule rather than per PR, so drift is caught even when nobody is pushing.
• It needs the shared test account, so mind the existing rate-limiting constraints.

Definition of done

• A schema change on YTM's side produces a clear failure naming the model and the field.
• Normal A/B variation does not produce failures.