feat: 完善多模型配置与运行切换

将模型连接统一持久化管理，并支持 Agent 运行中切换模型以及中断后选择模型继续。补充配置校验、事务与前后端交互测试。

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

@@ -60,4 +60,16 @@
             updated_at = CURRENT_TIMESTAMP
         WHERE id = #{id, typeHandler=tech.easyflow.manuagent.typehandler.UuidTypeHandler}
     </update>
+
+    <update id="setEnabled">
+        UPDATE app.model_config
+        SET enabled = #{enabled},
+            updated_at = CURRENT_TIMESTAMP
+        WHERE id = #{id, typeHandler=tech.easyflow.manuagent.typehandler.UuidTypeHandler}
+    </update>
+
+    <delete id="deleteModel">
+        DELETE FROM app.model_config
+        WHERE id = #{id, typeHandler=tech.easyflow.manuagent.typehandler.UuidTypeHandler}
+    </delete>
 </mapper>