fix(sqlserver): 表结构支持查看约束（主键/唯一/外键/检查/默认值） (#9581)

SQL Server 连接的「编辑表结构」面板里只有 DDL/字段/索引/外键/触发器，
没有「约束」页签；用户无法查看某张表的约束。

list_constraints 在 SQL Server 驱动中完全缺失：驱动侧没有对应实现，
dbx-core 的 list_constraints_core 没有 SQL Server 分支，直接落到默认的
空数组；前端 tableMetadataCapabilities 也没有声明 sqlserver 支持，
因此即使是空页签也不会渲染。

- dbx-drivers 新增 list_constraints，用一条 UNION ALL 查询汇总
  sys.key_constraints（PK/UNIQUE）、sys.foreign_keys（含引用库表列、
  ON UPDATE/DELETE 动作、is_not_trusted）、sys.check_constraints、
  sys.default_constraints；列名用 FOR XML PATH('') 聚合，兼容 SQL Server
  2008（无 STRING_AGG）
- list_constraints_core 增加 SQL Server 分支与链接服务器早退
- tableMetadataCapabilities 为 sqlserver 打开 constraints，前端显示「约束」页签
- 新增单测覆盖 SQL 形态与列名拆包，并补充前端能力断言

Co-authored-by: zipg <4047349+zipg@users.noreply.github.com>

crates/dbx-core/src/schema/mod.rs

@@ -7694,7 +7694,8 @@ pub async fn list_triggers_core(
 
 /// Lists structured constraints for a relation. Exposed generically so the
 /// agent protocol and native drivers share one route; the built-in drivers
-/// that implement it today are PostgreSQL, OpenGauss, and the Xugu agent.
+/// that implement it today are PostgreSQL, OpenGauss, SQL Server, and the
+/// Xugu agent.
 pub async fn list_constraints_core(
     state: &AppState,
     connection_id: &str,
@@ -7702,12 +7703,16 @@ pub async fn list_constraints_core(
     schema: &str,
     table: &str,
 ) -> Result<Vec<db::ConstraintInfo>, String> {
+    if crate::sql_dialect::parse_sqlserver_linked_schema_ref(schema).is_some() {
+        return Ok(vec![]);
+    }
     retry_metadata_connection(state, connection_id, Some(database), || async {
         let pool_key = state.get_or_create_metadata_pool_for_session(connection_id, Some(database), None).await?;
         let db_config = connection_config(state, connection_id).await;
 
         {
             let pool_handle = state.pool_handle(&pool_key).await;
+            try_sqlserver!(pool_handle, list_constraints, schema, table);
             if let Some(client) = extract_pool!(pool_handle.as_ref(), Agent) {
                 let mut client = client.lock().await;
                 return client