fix: clearing a wrong slot's MBID clears its source too, as tracks_mbid_source_pair requires (M498)

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

internal/library/duplicate_resolve_test.go

@@ -383,7 +383,7 @@ func TestResolveDuplicates_RemovesACopyFromAnotherSongsSlot_Integration(t *testi
 	seedTrackList(t, f.pool, f.album, map[string]int{"Void": 220500, "Reach": 281000})
 	// The wrong file is Void's recording, so it carries Void's MBID.
 	if _, err := f.pool.Exec(context.Background(),
-		`UPDATE tracks SET mbid = 'rec-void' WHERE id = ANY($1)`, []pgtype.UUID{f.a.ID, f.b.ID}); err != nil {
+		`UPDATE tracks SET mbid = 'rec-void', mbid_source = 'tag' WHERE id = ANY($1)`, []pgtype.UUID{f.a.ID, f.b.ID}); err != nil {
 		t.Fatal(err)
 	}
 	res, err := ResolveDuplicates(context.Background(), f.pool, nil, "", true)