The _DummyConfigManager stubs in test_metadata_service_musicmap.py and
test_metadata_service_artist_image.py were missing get_active_media_server(),
which the existing test_metadata_service_discography.py dummy provides.
Both files install their dummy via sys.modules["config.settings"] with an
"if not in sys.modules" guard, so whichever test file loads first wins.
When the new files load alphabetically before discography, the limited
dummy persists and later tests hit AttributeError on get_active_media_server.
Adds the same get_active_media_server method to both dummies so all three
test files are equivalent and test ordering no longer affects outcomes.
- Relocate the streamed MusicMap similar-artist flow out of web_server.py and into core.metadata_service.
- Match similar artists through the configured source-priority chain instead of assuming Spotify first.
- Add iTunes artwork fallback so streamed artist payloads still carry image_url when search results are sparse.
- Cover the new service behavior with tests.
