Fix test-order-dependent failures in new metadata_service tests

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.

tests/test_metadata_service_artist_image.py

@@ -30,6 +30,9 @@ if "config.settings" not in sys.modules:
         def get(self, key, default=None):
             return default
 
+        def get_active_media_server(self):
+            return "primary"
+
     settings_mod.config_manager = _DummyConfigManager()
     config_pkg.settings = settings_mod
     sys.modules["config"] = config_pkg

tests/test_metadata_service_musicmap.py

@@ -30,6 +30,9 @@ if "config.settings" not in sys.modules:
         def get(self, key, default=None):
             return default
 
+        def get_active_media_server(self):
+            return "primary"
+
     settings_mod.config_manager = _DummyConfigManager()
     config_pkg.settings = settings_mod
     sys.modules["config"] = config_pkg