Allow query parameters in LibrarySection.hubs() and add Playlist.centroid

[ajacobson]

Description

Two small additions to support the personalized "Mixes For You" hub in music sections:

• LibrarySection.hubs() now accepts optional query parameters (e.g. count=10 to limit items per hub, or includeMyMixes=1 to include the "Mixes For You" hub), matching the signature Library.hubs() already has. includeStations=1 remains the default via setdefault, so the request is unchanged for existing callers (including MusicSection.stations()), and it can now be overridden.

• New Playlist.centroid property returning the centroid Artist a personalized "Mix For You" playlist is built around, or None for regular playlists. Mix playlists have no composite/thumb of their own — the nested centroid <Directory> is their only artwork source, and there was previously no public way to reach it.

• 'centroid' is added to _DONT_RELOAD_FOR_KEYS: it is legitimately None on every regular playlist, so without the exemption, accessing it on a partial object would trigger a spurious auto-reload (which can never surface a centroid anyway — it only exists in the hub payload). The mocked test asserts zero reloads.

Since the CI bootstrap server cannot generate personalized mixes, test_Playlist_centroid replays a hubs response captured from a live PMS (attribute shapes preserved: mixes carry no ratingKey, and artwork lives only on the nested centroid <Directory>).

Tested against a live PMS: default hubs() output unchanged across section types, MusicSection.stations() unaffected, count/include flags honored on movie/show/music sections, and 9 real mixes parsing with centroid artist + thumb with no extra requests.

Motivation: music-assistant/server#3736, where the downstream Plex provider currently maintains a manual-parsing workaround for these items and the maintainers asked for an upstream fix.

Type of change

• New feature (non-breaking change which adds functionality)

Checklist:

• My code follows the style guidelines of this project
• I have performed a self-review of my own code
• I have commented my code, particularly in hard-to-understand areas
• I have added or updated the docstring for new or existing methods
• I have added tests when applicable

[Copilot]

Pull request overview

This PR extends PlexAPI’s library hub querying and adds support for parsing “Mixes For You” music mix playlists by exposing the centroid artist embedded in hubs payloads.

Changes:

• Add **kwargs passthrough to LibrarySection.hubs() (with includeStations=1 preserved as the default) to support parameters like count and includeMyMixes.
• Introduce Playlist.centroid to expose the centroid Artist for personalized mix playlists (or None for regular playlists).
• Add tests and a captured hubs payload fixture to validate centroid parsing and to ensure no auto-reload occurs when centroid is absent.

Reviewed changes

Copilot reviewed 6 out of 6 changed files in this pull request and generated no comments.

Show a summary per file

File	Description
tests/test_playlist.py	Adds coverage for Playlist.centroid parsing from hubs payloads and verifies no unintended reload for regular/partial playlists.
tests/test_library.py	Adds a regression test validating LibrarySection.hubs(count=...) limits hub item payload sizes.
tests/payloads.py	Adds a recorded hubs XML payload representing “Mixes For You” mix + centroid directory structure.
plexapi/playlist.py	Implements Playlist.centroid as a cached data property returning the centroid Artist when present.
plexapi/library.py	Updates LibrarySection.hubs() to accept query kwargs while keeping prior behavior via default includeStations=1.
plexapi/base.py	Adds centroid to the “don’t auto-reload when None” key set to prevent spurious reload attempts for hub-only centroid data.

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