End dashboard sessions with a warning when the receiver dies unexpectedly (#5157)

# What does this implement/fix?

A dashboard cast session only ended via an explicit `dashboard/hide`.
When the receiver app on the device died for any other reason (network
hiccup leading to an app quit, user exits the app on the device, another
app cast over it, an automation stopping the player), the session
lingered forever: nothing was logged and the frontend kept showing the
endpoint as "casting".

- Add `DashboardController.end_session(dashboard_id, reason)`: drops the
session, logs a warning (`Dashboard session on <name> ended: <reason>`)
and signals `DASHBOARD_SESSIONS_UPDATED`, so the frontend's casting
indicator clears itself
- The chromecast provider now watches devices it casts to: a cast status
where the receiver app is gone, replaced or restarted ends the session
immediately; a lost device connection ends it after a 60s grace period
- Same-app audio playback keeps the session alive (cast session id
unchanged), and a connection blip that recovers re-verifies the app
instead of ending the session
- Add tests for the controller method and the provider liveness
detection

**Related issue (if applicable):**

- related issue N/A

## 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.

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Co-authored-by: Marvin Schenkel <[email protected]>