refactor(message): implement atomic message merging and patching to prevent race conditions

Introduce `mergeMessages` and `patchMessages` to the message store to handle
updates atomically within Zustand's `set` callback. This replaces manual
read-modify-write patterns in services, preventing message loss during
concurrent WebSocket updates and synchronization processes.

- Add `mergeMessages` for atomic merging of new messages into existing lists
- Add `patchMessages` for efficient field updates (e.g., sender info, status)
- Update `MessageSendService`, `MessageSyncService`, and `WSMessageHandler`
  to use these new atomic operations
- Remove reliance on external `mergeMessagesById` in service layers to
  ensure state consistency

src/stores/message/store.ts

@@ -84,6 +84,8 @@ export interface MessageActions {
   updateConversation: (conversation: ConversationResponse) => void;
   removeConversation: (conversationId: string) => void;
   setMessages: (conversationId: string, messages: MessageResponse[]) => void;
+  mergeMessages: (conversationId: string, incoming: MessageResponse[]) => void;
+  patchMessages: (conversationId: string, patches: Map<string, Partial<MessageResponse>>) => void;
   setUnreadCount: (total: number, system: number) => void;
   setLastSystemMessageAt: (time: string | null) => void;
   setSSEConnected: (connected: boolean) => void;
@@ -331,6 +333,48 @@ export const useMessageStore = create<MessageStore>((set, get) => ({
     });
   },
 
+  /**
+   * 原子性合并消息到 messagesMap
+   * 在 zustand set 回调内完成 read-merge-write，消除并发竞态
+   */
+  mergeMessages: (conversationId: string, incoming: MessageResponse[]) => {
+    if (incoming.length === 0) return;
+    const normalizedId = normalizeConversationId(conversationId);
+    set(state => {
+      const existing = state.messagesMap.get(normalizedId) || [];
+      const merged = mergeMessagesById(existing, incoming);
+      const newMessagesMap = new Map(state.messagesMap);
+      newMessagesMap.set(normalizedId, merged);
+      return { messagesMap: newMessagesMap };
+    });
+  },
+
+  /**
+   * 原子性 patch：按消息 ID 更新指定字段（如 sender、status）
+   * 不影响其他消息，不丢失并发写入
+   */
+  patchMessages: (conversationId: string, patches: Map<string, Partial<MessageResponse>>) => {
+    if (patches.size === 0) return;
+    const normalizedId = normalizeConversationId(conversationId);
+    set(state => {
+      const existing = state.messagesMap.get(normalizedId);
+      if (!existing) return state;
+      let changed = false;
+      const updated = existing.map(m => {
+        const patch = patches.get(String(m.id));
+        if (patch) {
+          changed = true;
+          return { ...m, ...patch };
+        }
+        return m;
+      });
+      if (!changed) return state;
+      const newMessagesMap = new Map(state.messagesMap);
+      newMessagesMap.set(normalizedId, updated);
+      return { messagesMap: newMessagesMap };
+    });
+  },
+
   setUnreadCount: (total: number, system: number) => {
     set({ totalUnreadCount: total, systemUnreadCount: system });
   },