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/session/manager.test.ts

@@ -5,6 +5,8 @@
  *
  * 注意：SessionManager 使用动态 import 加载 @ai-assistant/core。
  * 测试中会创建新实例并记录初始会话数量，以确保测试独立性。
+ *
+ * 消息存储已移至 Core 层，Server 的 SessionManager 只负责会话元数据管理。
  */
 
 import { describe, it, expect, beforeEach, afterEach, vi } from 'vitest';
@@ -172,16 +174,6 @@ describe('SessionManager', () => {
       expect(result).toBe(false);
     });
 
-    it('删除会话时同时删除消息', async () => {
-      // 注意：这个测试会自己删除 session，不需要追踪
-      const session = await manager.create({ name: 'With Messages' });
-      await manager.addMessage(session.id, { role: 'user', content: 'Hello' });
-
-      await manager.delete(session.id);
-
-      expect(manager.getMessages(session.id)).toEqual([]);
-    });
-
     it('删除后 count 减少', async () => {
       // 注意：这个测试会自己删除 session，不需要追踪
       const session = await manager.create({ name: 'Test' });
@@ -232,112 +224,28 @@ describe('SessionManager', () => {
     });
   });
 
-  describe('getMessages - 获取消息', () => {
-    it('返回会话消息', async () => {
-      const session = await createTrackedSession({ name: 'Test' });
-      await manager.addMessage(session.id, { role: 'user', content: 'Hello' });
-      await manager.addMessage(session.id, { role: 'assistant', content: 'Hi!' });
-
-      const messages = manager.getMessages(session.id);
-
-      expect(messages.length).toBe(2);
-      expect(messages[0].content).toBe('Hello');
-      expect(messages[1].content).toBe('Hi!');
-    });
-
-    it('新会话消息为空', async () => {
-      const session = await createTrackedSession({ name: 'Test' });
-      const messages = manager.getMessages(session.id);
-      expect(messages).toEqual([]);
-    });
-
-    it('不存在的会话返回空数组', () => {
-      const messages = manager.getMessages('non-existent-id-12345');
-      expect(messages).toEqual([]);
-    });
-  });
-
-  describe('addMessage - 添加消息', () => {
-    it('添加用户消息', async () => {
-      const session = await createTrackedSession({ name: 'Test' });
-      const message = await manager.addMessage(session.id, {
-        role: 'user',
-        content: 'Hello',
-      });
-
-      expect(message).toBeDefined();
-      expect(message?.role).toBe('user');
-      expect(message?.content).toBe('Hello');
-      expect(message?.id).toBeDefined();
-      expect(message?.sessionId).toBe(session.id);
-    });
-
-    it('添加助手消息', async () => {
-      const session = await createTrackedSession({ name: 'Test' });
-      const message = await manager.addMessage(session.id, {
-        role: 'assistant',
-        content: 'Hello!',
-      });
-
-      expect(message?.role).toBe('assistant');
-    });
-
-    it('添加系统消息', async () => {
-      const session = await createTrackedSession({ name: 'Test' });
-      const message = await manager.addMessage(session.id, {
-        role: 'system',
-        content: 'System prompt',
-      });
-
-      expect(message?.role).toBe('system');
-    });
-
-    it('消息有唯一 ID', async () => {
-      const session = await createTrackedSession({ name: 'Test' });
-      const msg1 = await manager.addMessage(session.id, { role: 'user', content: 'Hello' });
-      const msg2 = await manager.addMessage(session.id, { role: 'assistant', content: 'Hi!' });
-
-      expect(msg1?.id).not.toBe(msg2?.id);
-    });
-
-    it('消息有正确的时间戳', async () => {
-      const session = await createTrackedSession({ name: 'Test' });
-      const before = new Date().toISOString();
-      const message = await manager.addMessage(session.id, { role: 'user', content: 'Hello' });
-      const after = new Date().toISOString();
-
-      expect(message?.createdAt).toBeDefined();
-      expect(message?.createdAt! >= before).toBe(true);
-      expect(message?.createdAt! <= after).toBe(true);
-    });
-
-    it('更新会话的 messageCount', async () => {
+  describe('updateMessageCount - 更新消息计数', () => {
+    it('更新消息计数', async () => {
       const session = await createTrackedSession({ name: 'Test' });
       expect(session.messageCount).toBe(0);
 
-      await manager.addMessage(session.id, { role: 'user', content: 'Hello' });
-
-      const updated = manager.get(session.id);
-      expect(updated?.messageCount).toBe(1);
+      const updated = manager.updateMessageCount(session.id, 5);
+      expect(updated?.messageCount).toBe(5);
     });
 
-    it('更新会话的 updatedAt', async () => {
+    it('更新时更新 updatedAt', async () => {
       const session = await createTrackedSession({ name: 'Test' });
       const originalUpdatedAt = session.updatedAt;
 
       await new Promise((resolve) => setTimeout(resolve, 10));
-      await manager.addMessage(session.id, { role: 'user', content: 'Hello' });
+      manager.updateMessageCount(session.id, 3);
 
       const updated = manager.get(session.id);
       expect(updated?.updatedAt).not.toBe(originalUpdatedAt);
     });
 
-    it('不存在的会话返回 undefined', async () => {
-      const result = await manager.addMessage('non-existent-id-12345', {
-        role: 'user',
-        content: 'Hello',
-      });
-
+    it('不存在的会话返回 undefined', () => {
+      const result = manager.updateMessageCount('non-existent-id-12345', 10);
       expect(result).toBeUndefined();
     });
   });
@@ -410,48 +318,30 @@ describe('SessionManager', () => {
     });
   });
 
+  describe('getStorage 和 getProjectId - Storage 访问', () => {
+    it('getStorage 返回 Storage 实例或 null', () => {
+      const storage = manager.getStorage();
+      // 可能为 null（如果 Core 未加载）或者是 SessionStorage 实例
+      expect(storage === null || typeof storage === 'object').toBe(true);
+    });
+
+    it('getProjectId 返回字符串', async () => {
+      const session = await createTrackedSession({ name: 'Test' });
+      const projectId = manager.getProjectId(session.id);
+      expect(typeof projectId).toBe('string');
+    });
+
+    it('不存在的会话返回默认 projectId', () => {
+      const projectId = manager.getProjectId('non-existent-id');
+      expect(typeof projectId).toBe('string');
+    });
+  });
+
   describe('边界情况', () => {
     it('处理特殊字符的会话名称', async () => {
       const session = await createTrackedSession({ name: '测试会话 <>&"\'`' });
       expect(session.name).toBe('测试会话 <>&"\'`');
     });
-
-    it('处理长消息内容', async () => {
-      const session = await createTrackedSession({ name: 'Test' });
-      const longContent = 'x'.repeat(10000);
-      const message = await manager.addMessage(session.id, {
-        role: 'user',
-        content: longContent,
-      });
-
-      expect(message?.content).toBe(longContent);
-    });
-
-    it('处理空字符串消息', async () => {
-      const session = await createTrackedSession({ name: 'Test' });
-      const message = await manager.addMessage(session.id, {
-        role: 'user',
-        content: '',
-      });
-
-      expect(message?.content).toBe('');
-    });
-
-    it('多个会话独立的消息', async () => {
-      const session1 = await createTrackedSession({ name: 'Session 1' });
-      const session2 = await createTrackedSession({ name: 'Session 2' });
-
-      await manager.addMessage(session1.id, { role: 'user', content: 'Message for session 1' });
-      await manager.addMessage(session2.id, { role: 'user', content: 'Message for session 2' });
-
-      const messages1 = manager.getMessages(session1.id);
-      const messages2 = manager.getMessages(session2.id);
-
-      expect(messages1.length).toBe(1);
-      expect(messages2.length).toBe(1);
-      expect(messages1[0].content).toBe('Message for session 1');
-      expect(messages2[0].content).toBe('Message for session 2');
-    });
   });
 });