Use artist top tracks when sampling genre and dynamic radio seeds (#6155)

# What does this implement/fix?

Genre playback and Endless Mix artist/genre seeds resolved each sampled
artist through the `all_tracks` enqueue preference, walking the full
discography (throttled, uncached) only to keep 5 tracks per artist. They
now sample the bounded artist `top_tracks` listing, falling back to full
resolution when no provider offers one (local libraries, where it is
cheap); direct artist playback still honors the enqueue preference. This
affects nearly every streaming provider, since only phishin and qqmusic
declare `ARTIST_TRACKS`; the old walk cost roughly 25-100s per 50-album
artist depending on the provider's throttle (Tidal ~1 min, hence the
report), repaid on every dynamic refill.

**Related issue (if applicable):**

- related issue https://github.com/orgs/music-assistant/discussions/6276

## Types of changes

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- [x] Bugfix (non-breaking change which fixes an issue) — `bugfix`
- [ ] New feature (non-breaking change which adds functionality) —
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- [ ] Enhancement to an existing feature — `enhancement`
- [ ] New music/player/metadata/plugin provider — `new-provider`
- [ ] Breaking change (fix or feature that would cause existing
functionality to not work as expected) — `breaking-change`
- [ ] Refactor (no behaviour change) — `refactor`
- [ ] Documentation only — `documentation`
- [ ] Maintenance / chore — `maintenance`
- [ ] CI / workflow change — `ci`
- [ ] Dependencies bump — `dependencies`

## Checklist

- [x] The code change is tested and works locally.
- [x] `pre-commit run --all-files` passes.
- [x] `pytest` passes, and tests have been added/updated under `tests/`
where applicable.
- [ ] For changes to shared models, the companion PR in
`music-assistant/models` is linked.
- [ ] For changes affecting the UI, the companion PR in
`music-assistant/frontend` is linked.
- [x] I have read and complied with the project's [AI
Policy](https://github.com/music-assistant/.github/blob/main/AI_POLICY.md)
for any AI-assisted contributions.
- [ ] I have [raised a PR against the documentation
repository](https://github.com/music-assistant/music-assistant.io/blob/main/CONTRIBUTING.md)
targeting the main or beta branch as appropriate.

---------

Co-authored-by: Claude Fable 5 <[email protected]>