Quieter track changes: don't warn when the next track has to wait for a free Spotify slot (#6170)

# What does this implement/fix?

When the next track is prepared ahead of time (the prefetch that starts
~60 seconds before a track ends) while a single-stream provider such as
Spotify is still busy delivering the current track, that early attempt
fails on purpose and is retried at the right moment. The failed attempt
still produced two scary `WARNING ... The stdin feeder task ended with
error: Spotify is already playing something else` log lines at every
track change, even though nothing was wrong.

- log the stdin feeder ending with a provider stream-limit error at
debug level instead of warning; the retry mechanism already handles this
case
- all other feeder errors keep their warning
- added a test that pins the downgraded log level and verifies the error
still reaches the caller

**Related issue (if applicable):**

- related issue N/A

## Types of changes

- [ ] 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`
- [x] 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.