[Task] — `ShenyuClientHttpRegistryController` (client registration endpoint) has zero test

- severity: High; files: `shenyu-admin/src/main/java/org/apache/shenyu/admin/controller/ShenyuClientHttpRegistryController.java:45` (register-metadata/uri/apiDoc/discoveryConfig/mcp/offline endpoints); confirmed no `ShenyuClientHttpRegistryControllerTest` references the class
- description: HTTP registration endpoint accepting client registrations from all gateway clients; broken registration = no upstreams routed. `ShenyuHttpRegistryControllerTest` exists for a sibling but does not reference this class.
- suggested_fix: Add `@WebMvcTest` covering register/unregister/registerMetadata endpoints.

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_Identified during the 2026-08-02 deep re-scan; full list in [`docs/scan2-2026-08-02/00-consolidated-critical-high.md`](docs/scan2-2026-08-02/00-consolidated-critical-high.md)._

Start with shenyu-admin/src/main/java/org/apache/shenyu/admin/controller/ShenyuClientHttpRegistryController.java at line 45, then read ShenyuHttpRegistryControllerTest for the sibling controller's testing pattern. Add @WebMvcTest coverage for the register, unregister, and registerMetadata endpoints, and extend coverage to the listed registration endpoints as appropriate. Done means the controller has tests and no longer has zero test references.

Written by the indexing model from the issue text.