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

[bobbai00]

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

• Refactor / Cleanup
• DevOps / Deployment / CI
• Testing / QA
• Documentation
• Performance
• Other