feat: add validated continuations for get_playlist (#778)

[Locko2901]

Closes #778

Adds an opt-in validate_responses flag to get_playlist, same as
get_library_songs already has. When enabled, short continuation
pages are retried until complete.

• get_validated_continuations_2025 in continuations.py
• raises YTMusicUserError when neither trackCount nor limit is known
• added tests

[Copilot]

Pull request overview

This PR adds an opt-in retry/validation mechanism for playlist continuations to improve completeness when YouTube Music returns short continuation pages, and adds test coverage for the new continuation behavior and the new get_playlist(validate_responses=...) flag.

Changes:

• Added validate_responses flag to PlaylistsMixin.get_playlist, using a validated continuation strategy when enabled.
• Implemented get_validated_continuations_2025 to retry short continuation pages up to a retry limit.
• Added unit tests for validated continuations and an OAuth-backed playlist test for the new flag (plus an error-case test when trackCount is unavailable).

Reviewed changes

Copilot reviewed 4 out of 4 changed files in this pull request and generated 2 comments.

File	Description
ytmusicapi/mixins/playlists.py	Adds validate_responses parameter and routes playlist track continuation through validated retries when enabled.
ytmusicapi/continuations.py	Introduces validated continuation retrieval for the 2025 browse-continuation format and generalizes retry helper typing.
tests/test_continuations.py	Adds unit tests covering retry behavior, retry caps, and stopping at limit for validated continuations.
tests/mixins/test_playlists.py	Adds an OAuth-backed test for validating large playlist retrieval and a no-trackCount error case.

---

💡 Add Copilot custom instructions for smarter, more guided reviews. Learn how to get started.