Stop random music playing after a notification clip (#6075)

# What does this implement/fix?

Sending a short notification or TTS clip to a player with
`media_player.play_media` (without `announce`) played the clip fine, but
once it finished Music Assistant started playing a random track from
your library.

The clip was treated as a normal track, so Autoplay ("keep the music
going") saw a queue about to run dry and topped it up. With `announce:
true` the queue is never involved, which is why that variant behaved.

A plain URL with no artist or album tags is now recognised as a one-off
clip (sound effect), and Autoplay leaves those alone.

**Related issue (if applicable):**

- https://github.com/music-assistant/support/issues/6194

## Changes

- The builtin provider returns a sound effect for a plain URL that
carries no music tags.
- URLs that do carry artist/album tags still become tracks, radio
streams still become radio, and URLs saved in your library or used in a
playlist are unchanged.
- Tests for the new classification and for Autoplay leaving a sound
effect alone.

## Types of changes

- [x] Bugfix (non-breaking change which fixes an issue) — `bugfix`
- [ ] New feature (non-breaking change which adds functionality) —
`new-feature`
- [ ] 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.