Fix multiple (virtual) devices on the same host being merged.

[MarvinSchenkel]

Make sure that universal players with the same IP, but different MAC address do not get merged.

Related issue: music-assistant/support#5266

[Copilot]

Pull request overview

Prevents universal players that represent multiple instances of the same protocol domain on a single host (same IP) from being merged together, addressing the “virtual devices on one host get merged” issue.

Changes:

• Add a merge guard in _check_merge_universal_players to skip merging when both universal players already have linked protocols from the same domain.
• Add a regression test covering multiple squeezelite instances on the same IP with distinct MACs to ensure no merge occurs.

Reviewed changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated no comments.

File	Description
music_assistant/controllers/players/protocol_linking.py	Skips UP merge when both candidates share at least one linked protocol domain (prevents same-domain instance merging on shared IP).
tests/core/test_protocol_linking.py	Adds a regression test ensuring multiple same-domain protocol instances on one IP remain separate and keep their protocol links.