Mutation and action response models

[sigma67]

Part of #307. Base branch: ytmusicapi-2. Read the contributor guide from #982 before starting.

The mutating and action endpoints. These currently return the raw YTM response typed as JsonDict, str | JsonDict or similar, which tells a consumer nothing about what they actually got back.

Scope

• mixins/library.py — rate_song, rate_playlist, edit_song_library_status, subscribe_artist, subscribe_artists, unsubscribe_artists, remove_history_items
• mixins/playlists.py — create_playlist, edit_playlist, add_playlist_items, remove_playlist_items, delete_playlist, join_collaborative_playlist
• mixins/uploads.py — delete_upload_entity
• mixins/search.py — remove_search_suggestions

Watch out for

• Several return str | JsonDict: a plain id string on success, or the raw response. That union should become something a consumer can actually branch on.
• add_playlist_items returns different content depending on duplicates and whether source_playlist was used.
• These endpoints are the least covered by tests and the most likely to have undocumented shapes — expect to inspect real responses rather than rely on docstrings.
• add_history_item and upload_song return requests.Response and are explicitly out of scope for Type validation with pydantic #307.

Definition of done

• Each listed method returns a model or a documented, narrowable union.
• mypy --strict clean.