Add episode service with docs and async/sync test coverage

[jonathan343]

Overview

This PR adds support for Spotify episode endpoints and aligns integration test naming conventions.

Summary

• Add AsyncEpisodeService / EpisodeService with:
  • get(id, market=None)
  • get_saved(limit=20, offset=0, market=None)
• Wire client.episodes on async and sync clients.
• Add SavedEpisode model support and export updates.
• Add episode docs:
  • new docs/reference/services/episodes.md
  • service index/nav/client reference/README updates
• Add async/sync unit tests for episode service and client wiring.
• Add async/sync integration tests for episode operations.
• Standardize integration filenames to tests/_async/integration/test_<service>.py (sync generated accordingly).
• Add a changelog entry for episode service support.

Testing

Added unit and integration tests for the episode service operations

(spotify-sdk) $ uv run pytest -m integration 
================================ test session starts ================================
platform darwin -- Python 3.14.2, pytest-9.0.2, pluggy-1.6.0
rootdir: /Users/gytndd/dev/personal/spotify-sdk
configfile: pyproject.toml
plugins: anyio-4.12.1, httpx-0.36.0
collected 433 items / 424 deselected / 9 selected                                   

tests/_async/integration/test_episode.py ..                                   [ 22%]
tests/_async/integration/test_library.py ..                                   [ 44%]
tests/_async/integration/test_show.py ..                                      [ 66%]
tests/_sync/integration/test_episode.py .                                     [ 77%]
tests/_sync/integration/test_library.py .                                     [ 88%]
tests/_sync/integration/test_show.py .                                        [100%]

======================== 9 passed, 424 deselected in 10.64s =========================

References

• Get Episode
• Get User's Saved Episodes