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/stores/message/index.ts

@@ -12,8 +12,8 @@ export type { MessageState, MessageEvent, MessageSubscriber, MessageEventType }
 // 导出同步服务
 export { messageSyncService, MessageSyncService } from './MessageSyncService';
 
-// 导出WebSocket处理器
-export { webSocketMessageHandler, WebSocketMessageHandler } from './WebSocketMessageHandler';
+// 导出SSE处理器
+export { sseMessageHandler, SSEMessageHandler } from './SSEMessageHandler';
 
 // 导出已读管理器
 export { readReceiptManager, ReadReceiptManager } from './ReadReceiptManager';