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_test.go

@@ -19,7 +19,7 @@ func (f *fakeProvider) ID() string                         { return f.id }
 func (f *fakeProvider) DisplayName() string                { return f.display }
 func (f *fakeProvider) RequiresAPIKey() bool               { return f.requiresKey }
 func (f *fakeProvider) DefaultEnabled() bool               { return f.defaultOn }
-func (f *fakeProvider) Configure(s ProviderSettings) error { return nil }
+func (f *fakeProvider) Configure(_ ProviderSettings) error { return nil }
 
 // fakeAlbumProvider opts into the AlbumCoverProvider capability for
 // the capability-filter test.
@@ -27,7 +27,7 @@ type fakeAlbumProvider struct {
 	fakeProvider
 }
 
-func (f *fakeAlbumProvider) FetchAlbumCover(ctx context.Context, mbid string) ([]byte, error) {
+func (f *fakeAlbumProvider) FetchAlbumCover(_ context.Context, _ string) ([]byte, error) {
 	return []byte("img"), nil
 }