test(artwork): pin that a request never fetches or samples album art

resolveItemLocal's guard against the remote ExternalImageURL fetch and
the 2x2 grid had no coverage: deleting it left the whole suite green
while putting synchronous network calls on the request path.

The worker resolving the same playlist is asserted alongside, so the
spec cannot pass by simply resolving nothing.

core/artwork/resolve_test.go

@@ -10,6 +10,7 @@ import (
 	"net/http/httptest"
 	"os"
 	"path/filepath"
+	"sync/atomic"
 
 	"github.com/navidrome/navidrome/conf"
 	"github.com/navidrome/navidrome/conf/configtest"
@@ -507,6 +508,36 @@ var _ = Describe("resolveItem", func() {
 			Expect(res.extError).To(BeFalse())
 		})
 
+		// The request path must never reach the network nor sample album art synchronously. The
+		// worker resolving the same playlist is asserted alongside, so this cannot pass vacuously.
+		It("resolves a playlist locally without fetching remotely or building the grid", func() {
+			conf.Server.EnableM3UExternalAlbumArt = true
+			var hits atomic.Int32
+			srv := httptest.NewServer(http.HandlerFunc(func(w http.ResponseWriter, _ *http.Request) {
+				hits.Add(1)
+				w.WriteHeader(http.StatusNotFound)
+			}))
+			defer srv.Close()
+
+			plRepo := tests.CreateMockPlaylistRepo()
+			plRepo.SetData(model.Playlists{{ID: "pllocal", Name: "Playlist", ExternalImageURL: srv.URL}})
+			plRepo.TracksRepo = &tests.MockPlaylistTrackRepo{AlbumIDs: []string{"t1"}}
+			ds.MockedPlaylist = plRepo
+			item := model.ArtworkQueueItem{ItemKind: "pl", ItemID: "pllocal"}
+
+			res, err := resolveItemLocal(ctx, ds, ffm, item)
+			Expect(err).ToNot(HaveOccurred())
+			Expect(res.reader).To(BeNil(), "no local source, and the grid is worker-only")
+			Expect(hits.Load()).To(BeZero(), "a request must never reach the network")
+
+			worker, err := resolveItem(ctx, ds, ag, ffm, item, nil)
+			Expect(err).ToNot(HaveOccurred())
+			Expect(worker.reader).ToNot(BeNil())
+			defer worker.reader.Close()
+			Expect(worker.source).To(Equal("generated"), "the worker does build the grid")
+			Expect(hits.Load()).To(Equal(int32(1)), "and the worker does fetch")
+		})
+
 		It("treats an ExternalImageURL 500 as a transient failure and sets extError", func() {
 			conf.Server.EnableM3UExternalAlbumArt = true
 			folderRepo.result = nil // no grid tiles, so the external failure is what surfaces