fix(discover): 修复聊天频道ID获取逻辑

- 统一使用RTCManager.instance.currentChannelId获取频道ID
- 移除冗余的空值判断逻辑
- 确保加入、离开和发送消息功能使用一致的频道标识

lib/controller/discover/room_controller.dart

@@ -157,7 +157,7 @@ class RoomController extends GetxController {
 
   Future<void> joinChat(CurrentRole role) async {
     final data = {
-      'channelId': rtcChannel.value?.channelId,
+      'channelId': RTCManager.instance.currentChannelId,
       'seatNumber': role == CurrentRole.maleAudience ? 1 : 2,
       'isMicrophoneOn': role != CurrentRole.normalUser ? true : false,
       'isVideoOn':
@@ -181,7 +181,7 @@ class RoomController extends GetxController {
   }
 
   Future<void> leaveChat() async {
-    final data = {'channelId': rtcChannel.value?.channelId};
+    final data = {'channelId': RTCManager.instance.currentChannelId};
     final response = await _networkService.rtcApi.disconnectRtcChannel(data);
     if (response.data.isSuccess) {
       isLive = false;
@@ -222,8 +222,7 @@ class RoomController extends GetxController {
 
   /// 发送公屏消息
   Future<void> sendChatMessage(String content) async {
-    final channelName =
-        rtcChannel.value?.channelId ?? RTCManager.instance.currentChannelId;
+    final channelName = RTCManager.instance.currentChannelId;
 
     final result = await _messageService.sendMessage(
       content: content,