Tidy up the source lookup in the seek command (#6072)

# What does this implement/fix?

The seek command looked up the active source on the player's raw
attributes, while the nine
other equivalent lookups in the players controller all read the resolved
player state. A source
that only exists in the resolved state, such as a live external source
session, never matched,
so the check that refuses seeking on a source that cannot seek was
silently skipped.

Nothing user-visible changes today: the seek command hands off to a live
external source before
it ever reaches that check. This just removes the outlier so the check
is not quietly dead if
that ordering ever changes.

- `cmd_seek` now reads `player.state.source_list` /
`player.state.active_source`, matching its
  nine siblings in the same file.
- The seek support check right below it reads
`player.state.supported_features` too, matching the
  next/previous checks.

**Related issue (if applicable):**

- Spotted while working on
https://github.com/music-assistant/server/pull/6070

## 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.