feat(sql_workbench): add DM datasource type mapping and support check

[LordofAvernus]

User description

Summary

• Add DM (达梦) database type mapping in convertDBType(): maps "达梦(DM)" to ODC DialectType "DM"
• Add pkgConst.DBTypeDM to SupportDBType() whitelist for SQL workbench support
• Add table-driven unit tests for both convertDBType() and SupportDBType() functions

Fixes https://github.com/actiontech/dms-ee/issues/780

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Description

• 增加 "达梦(DM)" 数据库类型映射支持

• 更新 SupportDBType 白名单，增加 pkgConst.DBTypeDM

• 添加单元测试验证转换逻辑和支持检查

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Diagram Walkthrough

flowchart LR
  A["修改 convertDBType 支持达梦"]
  B["更新 SupportDBType 添加 DBTypeDM"]
  C["新增单元测试验证功能"]
  A -- "后续" --> B
  B -- "验证" --> C

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File Walkthrough

	Relevant files
Enhancement	sql_workbench_service.go 增加达梦数据库类型映射与支持检查                                                                                  internal/sql_workbench/service/sql_workbench_service.go 添加 "达梦(DM)" 映射分支于 switch 语句中 更新 SupportDBType 函数，增加 pkgConst.DBTypeDM 判断 +3/-1
Tests	sql_workbench_service_test.go 新增单元测试验证达梦支持功能                                                                                      internal/sql_workbench/service/sql_workbench_service_test.go 新增单元测试验证 convertDBType 转换逻辑 验证 SupportDBType 对 pkgConst.DBTypeDM 的处理 +55/-0

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