Fix Pocket Casts sync failing on episodes without a duration (#5397)

# What does this implement/fix?

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Noticed an error when I was syncing pocketcasts for the first time.

The Pocket Casts API sends an explicit null for duration and playedUpTo
when a feed omits them, so the dict defaults never applied and the
threshold comparison raised "'>' not supported between instances of
'NoneType' and 'int'". A TypeError is not a MusicAssistantError, so it
escaped the per-podcast error handling in the library sync and aborted
the whole sync task at the first such episode.

Coerce both fields at every place they are read, and cover the sync,
single-episode and resume-position paths with tests.

**Related issue (if applicable):**

- related issue <link to issue>

## 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]>