fix: split listenbrainz spotify artist credits

[gbPagano]

Description

Fixes ListenBrainz imports/submissions where Spotify-backed listens provide multiple artists only as a comma-separated artist_name plus spotify_artist_ids, without additional_info.artist_names.

Koito now infers separate artists only when the comma-split artist names match the number of Spotify artist IDs, avoiding cases like Tyler, The Creator.

Also fixes the fallback parsing of mbid_mapping.artist_mbids.

Related Issue(s)

N/A

Type of Change

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)
• Documentation update

How Has This Been Tested?

• Unit Tests
• Manual Testing (please describe steps)

Manual test fixture was generated locally with ListenBrainz-style records such as Pineapple StormTv, Sant, Tiago Mac, KVSH, Samhara, Lagum, and control cases like Tyler, The Creator.

Checklist

• I have performed a self-review of my own code
• I have made corresponding changes to the documentation
• I have ensured my changes generate no new warnings

AI/LLM Usage

• I disclose that 100% of the code in this PR is written by an LLM. (please estimate)
• I fully understand all changes made by LLM-generated code.
• I have not and will not use an LLM to write any part of this PR description or PR comments.

Screenshots (if applicable)

Before	After