refactor(storage): 统一消息存储到 Core 层

问题：Server 端只存储最终文本响应，工具调用的中间消息丢失。

解决方案：
- Agent.chat() 返回 ChatResult，包含完整消息链
- Server SessionManager 简化为只管理会话元数据
- 消息 API 改为从 Core Storage 读取
- 移除 Server 端的消息存储和 addMessage 方法

影响范围：
- core: Agent.chat() 返回类型变更
- server: SessionManager 接口变更，移除消息存储
- server: GET /sessions/:id/messages 从 Core 读取
- server: 移除 POST /sessions/:id/messages 端点

packages/server/tests/unit/ws.test.ts

@@ -2,6 +2,8 @@
  * WebSocket Handler 测试
  *
  * 测试 WebSocket 连接处理、消息路由、广播功能等
+ *
+ * 注意：消息存储已移至 Core 层，Server 的 WebSocket 只负责消息广播和路由
  */
 
 import { describe, it, expect, beforeEach, vi } from 'vitest';
@@ -9,7 +11,6 @@ import { describe, it, expect, beforeEach, vi } from 'vitest';
 // Create mock functions
 const mockExists = vi.fn();
 const mockUpdateStatus = vi.fn();
-const mockAddMessage = vi.fn();
 const mockGet = vi.fn();
 
 // Mock dependencies before imports
@@ -17,7 +18,6 @@ vi.mock('../../src/session/manager.js', () => ({
   getSessionManager: vi.fn(() => ({
     exists: mockExists,
     updateStatus: mockUpdateStatus,
-    addMessage: mockAddMessage,
     get: mockGet,
   })),
 }));
@@ -60,7 +60,6 @@ describe('WebSocket Handler', () => {
   beforeEach(() => {
     vi.clearAllMocks();
     mockExists.mockReturnValue(false);
-    mockAddMessage.mockReturnValue({ id: 'msg-1', role: 'user', content: '', timestamp: Date.now() });
   });
 
   describe('handleWebSocket - 连接处理', () => {
@@ -110,6 +109,8 @@ describe('WebSocket Handler', () => {
 
     it('处理 message 类型消息', async () => {
       const ws = createMockWSContext();
+      handleWebSocket(ws as any, 'session-1');
+
       const message = JSON.stringify({
         type: 'message',
         payload: { content: 'Hello AI' },
@@ -117,10 +118,11 @@ describe('WebSocket Handler', () => {
 
       await handleWebSocketMessage(ws as any, 'session-1', message);
 
-      expect(mockAddMessage).toHaveBeenCalledWith('session-1', {
-        role: 'user',
-        content: 'Hello AI',
-      });
+      // 应该广播 message_received 确认
+      expect(ws.send).toHaveBeenCalledWith(
+        expect.stringContaining('"type":"message_received"')
+      );
+      // 应该调用 processMessage
       expect(processMessage).toHaveBeenCalledWith('session-1', 'Hello AI');
     });
 
@@ -175,6 +177,7 @@ describe('WebSocket Handler', () => {
 
     it('处理 ArrayBuffer 数据', async () => {
       const ws = createMockWSContext();
+      handleWebSocket(ws as any, 'session-1');
 
       const message = { type: 'message', payload: { content: 'ArrayBuffer test' } };
       const encoder = new TextEncoder();
@@ -182,14 +185,13 @@ describe('WebSocket Handler', () => {
 
       await handleWebSocketMessage(ws as any, 'session-1', buffer);
 
-      expect(mockAddMessage).toHaveBeenCalledWith('session-1', {
-        role: 'user',
-        content: 'ArrayBuffer test',
-      });
+      expect(processMessage).toHaveBeenCalledWith('session-1', 'ArrayBuffer test');
     });
 
     it('空 content 处理正确', async () => {
       const ws = createMockWSContext();
+      handleWebSocket(ws as any, 'session-1');
+
       const message = JSON.stringify({
         type: 'message',
         payload: {},
@@ -197,24 +199,19 @@ describe('WebSocket Handler', () => {
 
       await handleWebSocketMessage(ws as any, 'session-1', message);
 
-      expect(mockAddMessage).toHaveBeenCalledWith('session-1', {
-        role: 'user',
-        content: '',
-      });
+      expect(processMessage).toHaveBeenCalledWith('session-1', '');
     });
 
     it('处理 Blob 数据', async () => {
       const ws = createMockWSContext();
+      handleWebSocket(ws as any, 'session-1');
 
       const message = { type: 'message', payload: { content: 'Blob test' } };
       const blob = new Blob([JSON.stringify(message)]);
 
       await handleWebSocketMessage(ws as any, 'session-1', blob);
 
-      expect(mockAddMessage).toHaveBeenCalledWith('session-1', {
-        role: 'user',
-        content: 'Blob test',
-      });
+      expect(processMessage).toHaveBeenCalledWith('session-1', 'Blob test');
     });
 
     it('处理非标准数据类型（转为字符串）', async () => {
@@ -229,21 +226,6 @@ describe('WebSocket Handler', () => {
       expect(cancelProcessing).toHaveBeenCalledWith('session-1');
     });
 
-    it('addMessage 返回 null 时不调用 processMessage', async () => {
-      const ws = createMockWSContext();
-      mockAddMessage.mockReturnValue(null);
-
-      const message = JSON.stringify({
-        type: 'message',
-        payload: { content: 'Test' },
-      });
-
-      await handleWebSocketMessage(ws as any, 'session-1', message);
-
-      expect(mockAddMessage).toHaveBeenCalled();
-      expect(processMessage).not.toHaveBeenCalled();
-    });
-
     it('处理 tool_response 类型消息（TODO 场景）', async () => {
       const ws = createMockWSContext();
       const message = JSON.stringify({