Stop logging an ordinary Spotify seek as a failure (#6015)

# What does this implement/fix?

Every seek of a Spotify track played through the Soloist backend logged
two scary-looking warnings:

```
WARNING [ffmpeg.NNN] The stdin feeder task ended with error: Spotify Soloist delivered
incomplete audio for spotify:track:... (reached 17287ms of 259946ms)
```

Nothing was actually wrong — the audio played fine. Seeking replaces the
outgoing audio channel with a fresh one, and the check that guards
against the engine starving us was still judging that replaced channel:
cut on purpose part-way through, so it always looked short.

Now a channel Music Assistant cuts itself is marked as such and skipped
by that check, so the warning is left for a channel that really was
starved.

- Mark a channel that a seek, a skip or a session teardown replaces
- Skip the completeness check for those, keep it for every other channel
- Tests for both directions, so an engine-driven boundary still reports

Verified on a Spotify rig: the warning appears on `dev` for an ordinary
seek and is gone here, with playback and seeking unchanged.

**Related issue (if applicable):**

- 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.