test(coverart): fix registry-wipe + stale provider signature

Two distinct pre-existing test bugs in the last failing cluster:

1. newTestEnricher() called resetRegistryForTests() itself, wiping the
   fake providers callers Register() right before calling it (its own
   doc says callers register first and reconcile() picks them up). The
   ~8 TestEnrichArtist_* failures (source stayed NULL, no thumb
   written) all stem from reconcile() seeing an empty registry. Remove
   the internal reset; make every caller that lacked one own the
   registry lifecycle explicitly (resetRegistryForTests + t.Cleanup):
   SidecarFound, NoSidecarNoMBID, AlreadySidecar_NoOp,
   DrainsNullSourceOnly.

2. apiTestAlbumProvider.FetchAlbumCover had a stale signature
   (context, string) predating the AlbumRef refactor; it no longer
   satisfied coverart.AlbumCoverProvider, so the p.(AlbumCoverProvider)
   capability assertion failed and TestAdminListCoverSources got
   supports=[]. Fix the param to coverart.AlbumRef.

Test-only. (A1/A2 were correct; they unmasked these. Any residual
album-enricher semantics failures will be root-caused from the next
clean run, not bundled speculatively.)

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

internal/api/admin_cover_sources_test.go

@@ -27,7 +27,7 @@ func (p *apiTestAlbumProvider) DisplayName() string                         { re
 func (p *apiTestAlbumProvider) RequiresAPIKey() bool                        { return false }
 func (p *apiTestAlbumProvider) DefaultEnabled() bool                        { return true }
 func (p *apiTestAlbumProvider) Configure(_ coverart.ProviderSettings) error { return nil }
-func (p *apiTestAlbumProvider) FetchAlbumCover(_ context.Context, _ string) ([]byte, error) {
+func (p *apiTestAlbumProvider) FetchAlbumCover(_ context.Context, _ coverart.AlbumRef) ([]byte, error) {
 	return []byte("img"), nil
 }