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

routes/api.php

@@ -23,7 +23,11 @@ Route::middleware('auth.jwt')->group(function () {
 
     Route::get('/sessions', [ChatSessionController::class, 'index']);
     Route::post('/sessions', [ChatSessionController::class, 'store']);
+    Route::get('/sessions/{session_id}', [ChatSessionController::class, 'show']);
     Route::post('/sessions/{session_id}/messages', [ChatSessionController::class, 'append']);
     Route::get('/sessions/{session_id}/messages', [ChatSessionController::class, 'listMessages']);
+    Route::get('/sessions/{session_id}/messages/{message_id}', [ChatSessionController::class, 'showMessage']);
     Route::patch('/sessions/{session_id}', [ChatSessionController::class, 'update']);
+    Route::post('/sessions/{session_id}/archive', [ChatSessionController::class, 'archive']);
+    Route::get('/sessions/{session_id}/sse', [\App\Http\Controllers\ChatSessionSseController::class, 'stream']);
 });