feat(listenbrainz): match collaboration top-songs via all credited artist MBIDs (#5670)

* refactor(agents): drop single Artist fields from Song, keep only Artists

Song now represents credited artists solely via the Artists slice; the
single Artist/ArtistMBID fields and the ArtistList passthrough are removed.
Equals continues to hash the whole value.

* refactor(lastfm,listenbrainz): build Song.Artists in built-in agents

Last.fm and ListenBrainz now populate the Artists slice directly. For
ListenBrainz top songs, all credited artist MBIDs are mapped (the combined
display name on the first credit plus MBID-only collaborators) instead of
keeping only the first MBID, feeding the matcher's per-MBID specificity.

* refactor(external): set Song.Artists in top-songs enrichment

getMatchingTopSongs now seeds an Artists entry from the known artist when a
song carries none, replacing the single Artist/ArtistMBID writes.

* refactor(plugins): fold single-artist SongRef into Song.Artists

SongRef keeps its single Artist/ArtistMBID fields as part of the plugin wire
contract; songRefToAgentSong now folds them into a one-element Artists list
when a plugin sends no artists array.

* refactor(matcher): read Song.Artists directly

groupQueries consumes s.Artists now that ArtistList is gone; test inputs
build the Artists slice.

* fix(matcher): keep MBID-only artists as identity signals

Review follow-up: an artist credited only by MBID (empty ID and name) was
dropped before resolution in four places, defeating the multi-MBID matching
path this PR adds.

- matcher.groupQueries: treat a non-empty MBID as a usable artist signal
- external.getMatchingTopSongs: backfill the primary credit's name/MBID when
  the agent left them empty
- plugins.songRefToAgentSong: fold a single-artist SongRef when only ArtistMBID
  is set (not just when Artist name is set)
- listenbrainz.topSongArtists: return nil instead of an empty-name placeholder
  when neither name nor MBIDs are present

* fix(external): only backfill top-song artist onto an unnamed credit

Review follow-up (codex P2): the previous backfill stamped the queried
artist's MBID onto Artists[0] whenever it was empty, even when that credit
already named a different (e.g. featured) artist — producing a mismatched
name+MBID pair that could mis-rank matches. Now only an unnamed first credit
is filled (it is, by construction, the queried artist); an already-named
credit is left untouched.

Signed-off-by: Deluan <deluan@navidrome.org>

---------

Signed-off-by: Deluan <deluan@navidrome.org>

plugins/metadata_agent.go

@@ -227,26 +227,29 @@ func (a *MetadataAgent) GetSimilarSongsByArtist(ctx context.Context, id, name, m
 	return callSimilarSongsPluginFunction[capabilities.SimilarSongsByArtistRequest](ctx, a.plugin, FuncGetSimilarSongsByArtist, capabilities.SimilarSongsByArtistRequest{ID: id, Name: name, MBID: mbid, Count: int32(count)})
 }
 
-// songRefToAgentSong converts a single SongRef to agents.Song
+// songRefToAgentSong converts a single SongRef to agents.Song. SongRef keeps the single
+// Artist/ArtistMBID fields as part of the plugin wire contract; when a plugin sends those instead
+// of the artists array, they are folded into a one-element Artists list here.
 func songRefToAgentSong(s capabilities.SongRef) agents.Song {
 	var artists []agents.Artist
-	if len(s.Artists) > 0 {
+	switch {
+	case len(s.Artists) > 0:
 		artists = make([]agents.Artist, len(s.Artists))
 		for i, a := range s.Artists {
 			artists[i] = agents.Artist{ID: a.ID, Name: a.Name, MBID: a.MBID}
 		}
+	case s.Artist != "" || s.ArtistMBID != "":
+		artists = []agents.Artist{{Name: s.Artist, MBID: s.ArtistMBID}}
 	}
 	return agents.Song{
-		ID:         s.ID,
-		Name:       s.Name,
-		MBID:       s.MBID,
-		ISRC:       s.ISRC,
-		Artist:     s.Artist,
-		ArtistMBID: s.ArtistMBID,
-		Artists:    artists,
-		Album:      s.Album,
-		AlbumMBID:  s.AlbumMBID,
-		Duration:   uint32(s.Duration * 1000),
+		ID:        s.ID,
+		Name:      s.Name,
+		MBID:      s.MBID,
+		ISRC:      s.ISRC,
+		Artists:   artists,
+		Album:     s.Album,
+		AlbumMBID: s.AlbumMBID,
+		Duration:  uint32(s.Duration * 1000),
 	}
 }
 

plugins/metadata_agent_test.go

@@ -119,7 +119,8 @@ var _ = Describe("MetadataAgent", Ordered, func() {
 			Expect(err).ToNot(HaveOccurred())
 			Expect(songs).To(HaveLen(3))
 			Expect(songs[0].Name).To(Equal("Similar to Yesterday #1"))
-			Expect(songs[0].Artist).To(Equal("The Beatles"))
+			Expect(songs[0].Artists).To(HaveLen(1))
+			Expect(songs[0].Artists[0].Name).To(Equal("The Beatles"))
 		})
 	})
 
@@ -343,11 +344,18 @@ var _ = Describe("songRefToAgentSong multi-artist", func() {
 			{Name: "Future", MBID: "m-future"},
 		}))
 	})
-	It("leaves Artists nil and keeps the single Artist when no Artists provided", func() {
+	It("folds the single Artist/ArtistMBID into a one-element Artists when no Artists provided", func() {
 		ref := capabilities.SongRef{Name: "Solo", Artist: "Drake", ArtistMBID: "m-drake"}
 		got := songRefToAgentSong(ref)
+		Expect(got.Artists).To(Equal([]agents.Artist{{Name: "Drake", MBID: "m-drake"}}))
+	})
+	It("folds an MBID-only single artist (empty name) so the MBID is not dropped", func() {
+		ref := capabilities.SongRef{Name: "Solo", ArtistMBID: "m-drake"}
+		got := songRefToAgentSong(ref)
+		Expect(got.Artists).To(Equal([]agents.Artist{{MBID: "m-drake"}}))
+	})
+	It("leaves Artists nil when neither Artists nor the single Artist/ArtistMBID are provided", func() {
+		got := songRefToAgentSong(capabilities.SongRef{Name: "Anon"})
 		Expect(got.Artists).To(BeNil())
-		Expect(got.Artist).To(Equal("Drake"))
-		Expect(got.ArtistMBID).To(Equal("m-drake"))
 	})
 })

plugins/sonic_similarity_adapter_test.go

@@ -43,7 +43,8 @@ var _ = Describe("SonicSimilarityPlugin", Ordered, func() {
 			Expect(err).ToNot(HaveOccurred())
 			Expect(results).To(HaveLen(3))
 			Expect(results[0].Song.Name).To(Equal("Similar to Yesterday #1"))
-			Expect(results[0].Song.Artist).To(Equal("The Beatles"))
+			Expect(results[0].Song.Artists).To(HaveLen(1))
+			Expect(results[0].Song.Artists[0].Name).To(Equal("The Beatles"))
 			Expect(results[0].Similarity).To(Equal(1.0))
 			Expect(results[1].Similarity).To(Equal(0.9))
 			Expect(results[2].Similarity).To(Equal(0.8))
@@ -68,7 +69,8 @@ var _ = Describe("SonicSimilarityPlugin", Ordered, func() {
 			Expect(err).ToNot(HaveOccurred())
 			Expect(results).To(HaveLen(3))
 			Expect(results[0].Song.Name).To(Equal("Path Yesterday to Tomorrow Never Knows #1"))
-			Expect(results[0].Song.Artist).To(Equal("The Beatles"))
+			Expect(results[0].Song.Artists).To(HaveLen(1))
+			Expect(results[0].Song.Artists[0].Name).To(Equal("The Beatles"))
 			Expect(results[0].Similarity).To(Equal(1.0))
 			Expect(results[1].Similarity).To(Equal(0.95))
 			Expect(results[2].Similarity).To(Equal(0.9))