Fix artist name splitting with semicolons and Vorbis multi-field handling

[OzGav]

1. music_assistant/helpers/tags.py:

• Fixed artists and album_artists properties to not split on semicolons when values are already in a list (multiple ARTIST fields from Vorbis)
• Added MB ID count check for singular ARTIST/ALBUMARTIST tags - if count is 1, don't split even on semicolons
• Added warning in _parse_vorbis_artist_tags() for non-standard ARTISTS/ALBUMARTISTS tags in Vorbis files
• Added proper comments with spec references

2. tests/core/test_tags.py:

• Added test_vorbis_multiple_artist_fields_semicolon_in_name - mock test for multiple ARTIST fields with semicolon
• Added test_flac_multiple_artist_fields_semicolon_e2e - end-to-end test with real FLAC file
• Added test_id3_artist_tag_semicolon_single_mbid - test for single ARTIST with semicolon and 1 MB ID
• Added test_id3_artist_tag_semicolon_multiple_mbids - test for ARTIST with semicolon and multiple MB IDs
• Added test_id3_albumartist_tag_semicolon_single_mbid - same for album artist

3. tests/fixtures/ArtistWithSemicolon.flac:

• New test fixture with multiple ARTIST fields, one containing ave;new