fix(subsonic): strip OpenSubsonic extensions from playlists for legacy clients

buildOSPlaylist was the only OpenSubsonic builder function missing the
LegacyClients guard, causing attributes like `validUntil` and `readonly`
to appear in playlist XML responses for legacy clients like DSub2000.
This caused a crash when DSub2000 tried to parse evaluated smart
playlists containing the `validUntil` attribute.

server/subsonic/playlists.go

@@ -159,6 +159,10 @@ func (api *Router) buildPlaylist(ctx context.Context, p model.Playlist) response
 }
 
 func buildOSPlaylist(ctx context.Context, p model.Playlist) *responses.OpenSubsonicPlaylist {
+	player, ok := request.PlayerFrom(ctx)
+	if ok && isClientInList(conf.Server.Subsonic.LegacyClients, player.Client) {
+		return nil
+	}
 	pls := responses.OpenSubsonicPlaylist{}
 
 	if p.IsSmartPlaylist() {

server/subsonic/playlists_test.go

@@ -128,6 +128,23 @@ var _ = Describe("buildPlaylist", func() {
 			})
 		})
 
+		Context("with legacy client", func() {
+			BeforeEach(func() {
+				conf.Server.Subsonic.LegacyClients = "legacy-client"
+				player := model.Player{Client: "legacy-client"}
+				ctx = request.WithPlayer(ctx, player)
+			})
+
+			It("returns all standard fields but no OpenSubsonic extensions", func() {
+				result := router.buildPlaylist(ctx, playlist)
+
+				Expect(result.Comment).To(Equal("Test comment"))
+				Expect(result.Owner).To(Equal("admin"))
+				Expect(result.Public).To(BeTrue())
+				Expect(result.OpenSubsonicPlaylist).To(BeNil())
+			})
+		})
+
 		Context("when no player in context", func() {
 			It("returns all fields", func() {
 				result := router.buildPlaylist(ctx, playlist)
@@ -213,6 +230,23 @@ var _ = Describe("buildPlaylist", func() {
 				Expect(result.ValidUntil).To(Equal(&validUntil))
 			})
 		})
+
+		Context("with legacy client", func() {
+			BeforeEach(func() {
+				conf.Server.Subsonic.LegacyClients = "legacy-client"
+				player := model.Player{Client: "legacy-client"}
+				ctx = request.WithPlayer(ctx, player)
+			})
+
+			It("returns standard fields but no OpenSubsonic extensions", func() {
+				result := router.buildPlaylist(ctx, playlist)
+
+				Expect(result.Comment).To(Equal("Test comment"))
+				Expect(result.Owner).To(Equal("admin"))
+				Expect(result.Public).To(BeTrue())
+				Expect(result.OpenSubsonicPlaylist).To(BeNil())
+			})
+		})
 	})
 })