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/MessageStateManager.ts

@@ -31,7 +31,7 @@ export interface MessageState {
   conversationList: Conversation[];
   messagesMap: Map<string, Message[]>;
   unreadCount: UnreadCount;
-  isWebSocketConnected: boolean;
+  isSSEConnected: boolean;
   currentConversationId: string | null;
   isLoading: boolean;
   typingUsersMap: Map<string, string[]>;
@@ -50,7 +50,7 @@ export class MessageStateManager {
       conversationList: [],
       messagesMap: new Map(),
       unreadCount: { total: 0, system: 0 },
-      isWebSocketConnected: false,
+      isSSEConnected: false,
       currentConversationId: null,
       isLoading: false,
       typingUsersMap: new Map(),
@@ -86,7 +86,7 @@ export class MessageStateManager {
   }
 
   isConnected(): boolean {
-    return this.state.isWebSocketConnected;
+    return this.state.isSSEConnected;
   }
 
   getTypingUsers(groupId: string): string[] {
@@ -188,8 +188,8 @@ export class MessageStateManager {
     this.notify({ type: 'unread_count_updated', payload: count });
   }
 
-  setWebSocketConnected(connected: boolean): void {
-    this.state.isWebSocketConnected = connected;
+  setSSEConnected(connected: boolean): void {
+    this.state.isSSEConnected = connected;
     this.notify({ type: 'connection_changed', payload: connected });
   }
 
@@ -287,7 +287,7 @@ export class MessageStateManager {
       conversationList: [],
       messagesMap: new Map(),
       unreadCount: { total: 0, system: 0 },
-      isWebSocketConnected: false,
+      isSSEConnected: false,
       currentConversationId: null,
       isLoading: false,
       typingUsersMap: new Map(),