Explore, charts and mood models

[sigma67]

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

Scope

• get_explore and get_charts (mixins/explore.py, mixins/charts.py, parsers/explore.py)
• get_mood_categories and get_mood_playlists

Watch out for

Charts are heavily A/B tested server-side and the carousel contents vary by country — is_playlist_carousel and is_artist_carousel in mixins/charts.py exist precisely because of that (see #970). The models need to tolerate that variation rather than assume one layout.

get_charts(country=...) returns different sections per country. Test with at least ZZ and one specific country.

Definition of done

• Both carousel classifications produce correctly typed results.
• mypy --strict clean.