Plex: fix bugs, remove dead code and reduce repetition (#4179)

# What does this implement/fix?

<!-- Quick description and explanation of changes. -->
- Fix StreamDetails duration being passed in milliseconds instead of
seconds
- Translate plexapi NotFound into MediaNotFoundError instead of leaking
it
- Fix library-list cache key mismatch between read and write
- Don't send the local-auth sentinel as a real token; honor verify-cert
in unauthenticated connections during config flow
- Use removeprefix for collection ids instead of str.replace
- Remove unused _search_*_advanced methods, dead chapters/availability
code and the redundant type dispatch in recommendations()
- Add constants.py (config keys, sentinels, error messages)
- Add get_thumbnail_images/get_favorite_from_rating helpers to
deduplicate the parsers


## 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 targeting
the main or beta branch as appropriate.

Author: anatosun