main: 增强会话功能，支持归档与消息检索

- 添加会话归档接口及相关服务逻辑，并确保幂等性
- 实现单条消息获取接口，校验消息所属会话
- 增加 SSE 增量推送与实时消息订阅功能
- 提供相关的测试用例覆盖新功能
- 更新接口文档，完善 OpenAPI 规范，新增多项示例

docs/ChatSession/chat-session-openapi.yaml

@@ -111,6 +111,124 @@ paths:
                 $ref: '#/components/schemas/Error'
         "401":
           description: 未授权
+  /sessions/{session_id}/messages/{message_id}:
+    get:
+      tags: [ChatSession]
+      summary: 获取单条消息（校验 session_id）
+      security:
+        - bearerAuth: []
+      parameters:
+        - in: path
+          name: session_id
+          required: true
+          schema:
+            type: string
+            format: uuid
+        - in: path
+          name: message_id
+          required: true
+          schema:
+            type: string
+            format: uuid
+      responses:
+        "200":
+          description: 消息详情
+          content:
+            application/json:
+              schema:
+                $ref: '#/components/schemas/MessageResource'
+        "401":
+          description: 未授权
+        "404":
+          description: 未找到或不属于该会话
+  /sessions/{session_id}/archive:
+    post:
+      tags: [ChatSession]
+      summary: 归档会话（设为 CLOSED，幂等）
+      security:
+        - bearerAuth: []
+      parameters:
+        - in: path
+          name: session_id
+          required: true
+          schema:
+            type: string
+            format: uuid
+      responses:
+        "200":
+          description: 归档成功（或已归档）
+          content:
+            application/json:
+              schema:
+                $ref: '#/components/schemas/ChatSession'
+        "401":
+          description: 未授权
+        "404":
+          description: 未找到
+  /sessions/{session_id}/sse:
+    get:
+      tags: [ChatSession]
+      summary: SSE 增量推送（backlog + Redis 实时）
+      security:
+        - bearerAuth: []
+      parameters:
+        - in: path
+          name: session_id
+          required: true
+          schema:
+            type: string
+            format: uuid
+        - in: query
+          name: after_seq
+          schema:
+            type: integer
+            default: 0
+          description: backlog 起始 seq（若有 Last-Event-ID 以其为准）
+        - in: query
+          name: limit
+          schema:
+            type: integer
+            default: 200
+            maximum: 500
+      responses:
+        "200":
+          description: text/event-stream SSE 流
+          content:
+            text/event-stream:
+              schema:
+                type: string
+                example: |
+                  id: 1
+                  event: message
+                  data: {"message_id":"...","seq":1}
+        "401":
+          description: 未授权
+        "404":
+          description: 未找到
+  /sessions/{session_id}:
+    get:
+      tags: [ChatSession]
+      summary: 获取会话详情
+      security:
+        - bearerAuth: []
+      parameters:
+        - in: path
+          name: session_id
+          required: true
+          schema:
+            type: string
+            format: uuid
+      responses:
+        "200":
+          description: 会话详情
+          content:
+            application/json:
+              schema:
+                $ref: '#/components/schemas/ChatSession'
+        "401":
+          description: 未授权
+        "404":
+          description: 未找到
     get:
       tags: [ChatSession]
       summary: 按 seq 增量查询消息