Podcast and episode models

[sigma67]

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

Scope

mixins/podcasts.py, parsed by ytmusicapi/parsers/podcasts.py:

• get_channel and get_channel_episodes
• get_podcast
• get_episode
• get_episodes_playlist

Watch out for

Episodes carry fields songs do not (date, duration, description, podcast reference) and lack several a song has, so this is likely its own model rather than a reuse of the song model from #983.

Some podcast search results come back without a browseId (#976) — whatever the equivalent is here needs to be optional rather than assumed present.

Definition of done

• All five methods return models.
• Tests cover an episode with and without a description.
• mypy --strict clean.