Enrich matched playlist entries with provider metadata during import (#5554)

# What does this implement/fix?

<!-- Quick description and explanation of changes. -->

When importing a plain M3U playlist without #EXTPROV tags, the track
matcher only rewrote the entry's path to the matched URI without storing
any provider mappings. Resolving such an entry then produced a track
with an empty provider_mappings set, leaving it unplayable until the
startup migration repaired it on the next restart.

Build the full M3U entry (provider mappings, artists, album, images)
from the matched URI right away, falling back to storing just the URI
when the item can't be fetched.

**Related issue (if applicable):**

- related issue https://github.com/music-assistant/support/issues/6036

## 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) —
`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`
- [ ] 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.

Co-authored-by: Claude <[email protected]>