[Task] — Critical admin service impls with zero test (beyond T1/T2)

- severity: High/Medium; files (all under `shenyu-admin/src/main/java/org/apache/shenyu/admin/`):
- `service/impl/DiscoveryServiceImpl.java:69` (High — discovery CRUD + upstream sync)
- `service/impl/RegistryServiceImpl.java:41` (High — registration/publish paths)
- `service/impl/AiProxyConnectionServiceImpl.java:42` (Medium)
- `service/impl/OperationRecordLogServiceImpl.java:37` (Medium — audit log)
- `service/impl/NamespaceUserServiceImpl.java:38` (Medium)
- `service/manager/impl/RegisterApiDocServiceImpl.java:43` (Medium)
- `service/register/ShenyuClientRegisterMcpServiceImpl.java:48` (Medium)
- `service/impl/ScaleRuleServiceImpl.java` (Medium)
- `service/impl/ScalePolicyServiceImpl.java` (Medium)
- `shenyu-plugin-base/.../alert/AlarmServiceImpl.java` (Medium)
- description: Confirmed by content-grepping all test sources for each class name (not just filename matching; `*ServiceTest.java` variants excluded sibling impls that ARE covered). Sibling impls (`RuleServiceImpl`, `SelectorServiceImpl`, `PluginServiceImpl`, `AppAuthServiceImpl`, `MetaDataServiceImpl`, `DashboardUserServiceImpl`, `RoleServiceImpl`, `ResourceServiceImpl`, `PermissionServiceImpl`, `TagServiceImpl`, `ConfigsServiceImpl`, `SyncDataServiceImpl`, `DetailServiceImpl`, `FieldServiceImpl`) ARE covered — correctly excluded.
- suggested_fix: Add `*ServiceImplTest` for each.

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_Identified during the 2026-08-02 deep re-scan; full list in [`docs/scan2-2026-08-02/06-medium-tiers.md`](docs/scan2-2026-08-02/06-medium-tiers.md)._