Cheaper track changes: Spotify Soloist advertises its real single-stream limit (#6172)

# What does this implement/fix?

Follow-up to #6141 and #6170. The soloist engine plays one single-track
run at a time, but the provider still advertised librespot's two-slot
stream budget. The next-track prefetch (~60s before a track ends)
therefore passed the upfront slot check and spun up a full pipeline —
audio buffer, ffmpeg, engine request — twice per track change, only for
the engine to refuse it. #6170 silenced the warnings this produced; this
removes the wasted work itself.

- the stream-slot budget now follows the configured backend: one slot
for soloist (which is what the provider README already documented), and
three for librespot to cover two playing queues plus a prebuffer
- no new machinery: with a truthful slot count, the early prefetch now
costs a short quiet wait on the slot instead of a pipeline spawn, and
the retry fired once the current track has fully arrived takes the freed
slot, as before
- replaces the soloist module docstring that still described the
continuous-session design #6141 removed

**Related issue (if applicable):**

- related issue N/A

## 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.