refactor(agent-service): centralize backend DTOs, metadata, endpoint config, and auth

### Task Summary

Centralize the agent-service type/config layer (no runtime behavior change): backend request/response DTOs into `types/dto.ts`, operator-metadata types into `types/metadata.ts`, backend base URLs into `config/endpoints.ts`, and the JWT helpers into `auth/jwt.ts`. Replace `any` with precise types and consume the centralized types at the call sites; expand unit-test coverage.

Part of #5747. Implemented by #5928.

### Task Type

- [x] Refactor / Cleanup
- [ ] DevOps / Deployment / CI
- [x] Testing / QA
- [ ] Documentation
- [ ] Performance
- [ ] Other