fix(server/m7-cover-sources): forward-fix CI — unused-parameter lint

revive's unused-parameter rule flagged 7 test handler/method args:
- provider_test.go: fakeProvider.Configure(s) and
  fakeAlbumProvider.FetchAlbumCover(ctx, mbid) — interface
  conformance dummies that don't use their args.
- provider_mbcaa_test.go: 4 httptest handlers that ignore one or
  both of (w, r).
- provider_theaudiodb_test.go: the disabled-provider negative
  test's httptest handler.

All 7 renamed to _ per revive's convention.

internal/coverart/provider_theaudiodb_test.go

@@ -245,7 +245,7 @@ func TestTheAudioDB_DefaultsToTestKeyOnEmpty(t *testing.T) {
 }
 
 func TestTheAudioDB_DisabledReturnsNotFound(t *testing.T) {
-	srv := httptest.NewServer(http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
+	srv := httptest.NewServer(http.HandlerFunc(func(_ http.ResponseWriter, _ *http.Request) {
 		t.Error("server should not be hit when provider is disabled")
 	}))
 	defer srv.Close()