Clean up pending Sonos speaker setups when the provider is removed (#5480)

# What does this implement/fix?

When a Sonos speaker announces itself, the provider waits a few seconds
before setting it up, because those announcements arrive in bursts.
Unloading the provider did not cancel that pending setup, so a speaker
that announced itself just before the Sonos provider was removed still
got set up afterwards — connecting to the speaker and registering a
player for a provider that is no longer there.

Follow-up on #5433, which fixed the same class of leak per player but
left the provider level out of scope.

- Keep track of the player setups that are still waiting to run.
- Cancel them when the provider is unloaded, both while still pending
and once already started.
- Ignore speaker announcements that arrive once the provider is gone, so
a late one cannot start a new setup.
- Tests for each case.

**Related issue (if applicable):**

- n/a

## Types of changes

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- [ ] 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.