refactor: convert to nl2sql skills

backend/app/agent/nl2sql.py

@@ -191,14 +191,24 @@ def _set_cached_schema(source: str, connector: Any, schema: Dict[str, List[Dict[
     with _cache_lock:
         _schema_cache[key] = {"schema": schema, "expires_at": time.time() + SCHEMA_CACHE_TTL_SECONDS}
 
-def _check_connection_with_cache(source: str, connector: Any) -> bool:
+async def _check_connection_with_cache(source: str, connector: Any) -> bool:
     cache_key = _build_schema_cache_key(source, connector)
     now = time.time()
     with _cache_lock:
         cached = _connection_cache.get(cache_key)
         if cached and now < cached["expires_at"]:
             return bool(cached["ok"])
-    ok = connector.test_connection()
+    
+    # Run synchronous test_connection in a separate thread to avoid blocking event loop
+    try:
+        ok = await asyncio.wait_for(
+            asyncio.to_thread(connector.test_connection),
+            timeout=10.0
+        )
+    except Exception as e:
+        print(f"Connection test failed or timed out: {e}")
+        ok = False
+    
     with _cache_lock:
         _connection_cache[cache_key] = {"ok": ok, "expires_at": now + CONNECTION_CACHE_TTL_SECONDS}
     return ok
@@ -224,7 +234,7 @@ async def process_nl2sql(
     elif request.source == "upload":
         try:
             upload_started = time.perf_counter()
-            upload_payload = _get_upload_payload(request.file_url)
+            upload_payload = await asyncio.to_thread(_get_upload_payload, request.file_url)
             upload_df = upload_payload["df"]
             schema = upload_payload["schema"]
             await emit_progress(f"上传文件加载完成 ({time.perf_counter() - upload_started:.2f}s)")
@@ -234,14 +244,21 @@ async def process_nl2sql(
         try:
             ds_started = time.perf_counter()
             ds_id = int(request.source.split(":")[1])
-            db = SessionLocal()
-            try:
-                ds = db.query(DataSource).filter(DataSource.id == ds_id).first()
-                if not ds:
-                    return NL2SQLResponse(sql="", result=[], error=f"Data source not found: {request.source}")
-                connector = get_connector(ds)
-            finally:
-                db.close()
+            
+            def _get_ds_connector():
+                db = SessionLocal()
+                try:
+                    ds = db.query(DataSource).filter(DataSource.id == ds_id).first()
+                    if not ds:
+                        return None
+                    return get_connector(ds)
+                finally:
+                    db.close()
+            
+            connector = await asyncio.to_thread(_get_ds_connector)
+            if not connector:
+                return NL2SQLResponse(sql="", result=[], error=f"Data source not found: {request.source}")
+                
             await emit_progress(f"数据源配置读取完成 ({time.perf_counter() - ds_started:.2f}s)")
         except ValueError:
              return NL2SQLResponse(sql="", result=[], error=f"Invalid data source ID: {request.source}")
@@ -258,20 +275,35 @@ async def process_nl2sql(
             await emit_progress(f"命中 Schema 缓存，已加载 {len(schema)} 张表")
         else:
             conn_started = time.perf_counter()
-            if not _check_connection_with_cache(request.source, connector):
+            if not await _check_connection_with_cache(request.source, connector):
                 return NL2SQLResponse(sql="", result=[], error=f"Failed to connect to {request.source}")
             await emit_progress(f"连接检测完成 ({time.perf_counter() - conn_started:.2f}s)")
             schema_started = time.perf_counter()
-            schema = connector.get_schema()
+            try:
+                schema = await asyncio.wait_for(
+                    asyncio.to_thread(connector.get_schema),
+                    timeout=30.0
+                )
+            except Exception as e:
+                return NL2SQLResponse(sql="", result=[], error=f"Failed to fetch schema: {e}")
+            
             _set_cached_schema(request.source, connector, schema)
             await emit_progress(f"Schema 拉取完成，共 {len(schema)} 张表 ({time.perf_counter() - schema_started:.2f}s)")
             
     if connector and not schema:
          retry_started = time.perf_counter()
          # Double check in case schema was empty but connection is ok (e.g. empty db)
-         if not _check_connection_with_cache(request.source, connector):
+         if not await _check_connection_with_cache(request.source, connector):
               return NL2SQLResponse(sql="", result=[], error=f"Failed to connect to {request.source}")
-         schema = connector.get_schema()
+         
+         try:
+             schema = await asyncio.wait_for(
+                 asyncio.to_thread(connector.get_schema),
+                 timeout=30.0
+             )
+         except Exception as e:
+             return NL2SQLResponse(sql="", result=[], error=f"Failed to fetch schema on retry: {e}")
+             
          _set_cached_schema(request.source, connector, schema)
          await emit_progress(f"Schema 二次拉取完成，共 {len(schema)} 张表 ({time.perf_counter() - retry_started:.2f}s)")
          
@@ -282,7 +314,7 @@ async def process_nl2sql(
     if request.source.startswith("ds:"):
         try:
             ds_id = int(request.source.split(":")[1])
-            mdl = MDLService.get_mdl(ds_id)
+            mdl = await asyncio.to_thread(MDLService.get_mdl, ds_id)
             if mdl:
                 mdl_lines = ["\n### SEMANTIC MODEL (WrenMDL) ###"]
                 
@@ -392,7 +424,8 @@ Language: Chinese (Simplified)
         await emit_progress("正在执行 SQL 查询")
         if request.source == "upload":
             if upload_df is None:
-                upload_df = _get_upload_payload(request.file_url)["df"]
+                upload_payload = await asyncio.to_thread(_get_upload_payload, request.file_url)
+                upload_df = upload_payload["df"]
             timeout_stage = "sql_execution"
             formatted_results = await asyncio.wait_for(
                 asyncio.to_thread(_execute_upload_sql, sql_query, upload_df),