重构：使用 MyBatis-Flex 迁移应用 ORM

将应用自管表的 JdbcClient 数据访问迁移为实体、Mapper、构造器查询和必要的显式 SQL。

保留 AgentScope 自管表及原有业务语义，并补充事务、查询与数据库集成测试。

server/src/main/resources/mapper/ModelAssignmentMapper.xml

@@ -0,0 +1,18 @@
+<?xml version="1.0" encoding="UTF-8" ?>
+<!DOCTYPE mapper PUBLIC "-//mybatis.org//DTD Mapper 3.0//EN"
+        "https://mybatis.org/dtd/mybatis-3-mapper.dtd">
+<mapper namespace="tech.easyflow.manuagent.mapper.ModelAssignmentMapper">
+
+    <!-- 角色为主键，单语句 upsert 避免并发设置默认模型时出现先查后写竞态。 -->
+    <insert id="upsert">
+        INSERT INTO app.model_assignment(role, model_config_id, assigned_by)
+        VALUES (
+            #{assignment.role},
+            #{assignment.modelConfigId, typeHandler=tech.easyflow.manuagent.typehandler.UuidTypeHandler},
+            #{assignment.assignedBy, typeHandler=tech.easyflow.manuagent.typehandler.UuidTypeHandler})
+        ON CONFLICT (role) DO UPDATE SET
+            model_config_id = EXCLUDED.model_config_id,
+            assigned_by = EXCLUDED.assigned_by,
+            updated_at = CURRENT_TIMESTAMP
+    </insert>
+</mapper>