fix(lidarr): wrap MBID-lookup decode errors with ErrInvalidPayload + fill test gaps

- Decode errors in LookupArtist/AlbumByMBID now wrap ErrInvalidPayload so
  callers can errors.Is the same way they do against the M5a methods.
- Tighten TestLookupAlbumByMBID_BadJSON to assert on ErrInvalidPayload.
- Add empty-mbid rejection tests for both methods.
- Add TestLookupArtistByMBID_EmptyArrayReturnsErrNotFound (only the album
  variant had the test before — symmetric coverage now).
- Normalize the network-error test's api key to key123 for consistency
  with the rest of delete_test.go.

internal/lidarr/delete_test.go

@@ -86,8 +86,44 @@ func TestLookupAlbumByMBID_BadJSON(t *testing.T) {
 	defer srv.Close()
 
 	_, err := c.LookupAlbumByMBID(context.Background(), "x")
-	if err == nil {
-		t.Fatal("err = nil, want decode error")
+	if !errors.Is(err, ErrInvalidPayload) {
+		t.Errorf("err = %v, want ErrInvalidPayload", err)
+	}
+}
+
+func TestLookupAlbumByMBID_EmptyMBIDRejected(t *testing.T) {
+	c, srv := newTestClient(func(http.ResponseWriter, *http.Request) {
+		t.Error("server should not be called with empty mbid")
+	})
+	defer srv.Close()
+
+	if _, err := c.LookupAlbumByMBID(context.Background(), ""); err == nil {
+		t.Error("err = nil, want empty-mbid rejection")
+	}
+}
+
+func TestLookupArtistByMBID_EmptyArrayReturnsErrNotFound(t *testing.T) {
+	c, srv := newTestClient(func(w http.ResponseWriter, _ *http.Request) {
+		w.Header().Set("Content-Type", "application/json")
+		w.WriteHeader(http.StatusOK)
+		_, _ = w.Write([]byte("[]"))
+	})
+	defer srv.Close()
+
+	_, err := c.LookupArtistByMBID(context.Background(), "x")
+	if !errors.Is(err, ErrNotFound) {
+		t.Errorf("err = %v, want ErrNotFound", err)
+	}
+}
+
+func TestLookupArtistByMBID_EmptyMBIDRejected(t *testing.T) {
+	c, srv := newTestClient(func(http.ResponseWriter, *http.Request) {
+		t.Error("server should not be called with empty mbid")
+	})
+	defer srv.Close()
+
+	if _, err := c.LookupArtistByMBID(context.Background(), ""); err == nil {
+		t.Error("err = nil, want empty-mbid rejection")
 	}
 }
 
@@ -158,7 +194,7 @@ func TestDeleteAlbum_5xxReturnsErrServerError(t *testing.T) {
 func TestDeleteAlbum_NetworkErrorReturnsErrUnreachable(t *testing.T) {
 	srv := httptest.NewServer(http.HandlerFunc(func(http.ResponseWriter, *http.Request) {}))
 	srv.Close() // server is closed; client should fail to connect
-	c := NewClient(srv.URL, "test-key")
+	c := NewClient(srv.URL, "key123")
 
 	err := c.DeleteAlbum(context.Background(), 42, true, true)
 	if !errors.Is(err, ErrUnreachable) {