test(artwork): cover artist folder lookup for a single album without images

Add an e2e spec for an artist whose only album folder has no images of its
own, while the artist folder holds folder.jpg (plus unrelated images) and
ArtistArtPriority starts with folder.*. Before #5856, the album's parent was
promoted into the album paths, so the artist folder resolved to the library
root and the artist got no image. The spec fails if that promotion comes
back, and passes on current code.

Refs #5823

core/artwork/e2e/artist_test.go

@@ -90,6 +90,28 @@ var _ = Describe("Artist artwork resolution", func() {
 		})
 	})
 
+	When("the artist's only album folder has no images of its own", func() {
+		// Artist/
+		// ├── backdrop1.jpg
+		// ├── folder.jpg               ← matched by folder.*
+		// ├── logo.png
+		// └── Album/
+		//     ├── 01 - Track.mp3
+		//     └── 02 - Track.mp3
+		It("resolves the artist folder, not the library root", func() {
+			conf.Server.ArtistArtPriority = "folder.*, artist.*, album/artist.*"
+			setLayout(fstest.MapFS{
+				"Artist/Album/01 - Track.mp3": trackFile(1, "Track 1", map[string]any{"albumartist": "Artist", "album": "Album"}),
+				"Artist/Album/02 - Track.mp3": trackFile(2, "Track 2", map[string]any{"albumartist": "Artist", "album": "Album"}),
+				"Artist/backdrop1.jpg":        smallPNG("backdrop"),
+				"Artist/folder.jpg":           smallPNG("artist-folder"),
+				"Artist/logo.png":             smallPNG("logo"),
+			})
+			scan()
+			expectArtistFolder(soleArtist(), "Artist/folder.jpg")
+		})
+	})
+
 	When("the artist's only album has its tracks in disc subfolders", func() {
 		// Artist/
 		// ├── artist.jpg               ← wins (artist.* before album/artist.*)