refactor(messaging): replace WebSocket with SSE for real-time message handling

Replace WebSocket-based real-time communication with Server-Sent Events (SSE) across the messaging infrastructure. This includes:

- Create new SSEClient datasource to manage SSE connections
- Create new SSEMessageHandler to process SSE events
- Update ProcessMessageUseCase to use SSEClient instead of WebSocketClient
- Update MessageManager and MessageStateManager to work with SSE handlers
- Rename connection state variables from `isWebSocketConnected` to `isSSEConnected`
- Update type definitions in dto.ts (WSEventType → SSEEventType, etc.)
- Delete obsolete WebSocketClient and WebSocketMessageHandler files
- Update comments and documentation to reflect SSE terminology

This refactoring aligns with the backend's SSE implementation for better compatibility with React Native's networking capabilities.

src/types/dto.ts

@@ -672,28 +672,28 @@ export interface GroupAnnouncementListResponse {
   total_pages: number;
 }
 
-// ==================== WebSocket Event Types ====================
+// ==================== SSE Event Types ====================
 
-// WebSocket 事件类型
-export type WSEventType = 'message' | 'notice' | 'request' | 'meta';
+// SSE 事件类型
+export type SSEEventType = 'message' | 'notice' | 'request' | 'meta';
 
-// WebSocket 消息详细类型
-export type WSDetailType = 'private' | 'group' | 'follow' | 'like' | 'comment' | 'request' | 'typing' | 'heartbeat' | 'read';
+// SSE 消息详细类型
+export type SSEDetailType = 'private' | 'group' | 'follow' | 'like' | 'comment' | 'request' | 'typing' | 'heartbeat' | 'read';
 
-// WebSocket 事件消息段（与后端 message 字段一致）
-export interface WSSegment {
+// SSE 事件消息段（与后端 message 字段一致）
+export interface SSESegment {
   type: string;
   data: Record<string, any>;
 }
 
-// WebSocket 事件（后端推送的事件格式）
-export interface WSEvent {
+// SSE 事件（后端推送的事件格式）
+export interface SSEEvent {
   id: string;                    // 事件唯一ID
   time: number;                  // 事件时间戳（毫秒）
-  type: WSEventType;              // 事件类型: message(消息), notice(通知), request(请求), meta(元事件)
-  detail_type: WSDetailType;     // 详细类型: private(私聊), group(群聊), follow(关注), like(点赞)等
+  type: SSEEventType;              // 事件类型: message(消息), notice(通知), request(请求), meta(元事件)
+  detail_type: SSEDetailType;     // 详细类型: private(私聊), group(群聊), follow(关注), like(点赞)等
   seq: string;                   // 消息序号
-  message?: WSSegment[];         // 消息内容数组
+  message?: SSESegment[];         // 消息内容数组
   conversation_id: string;        // 会话ID
 }