Architecture hardening phase 2: extract resource generators

[fmenesesg]

Summary

Extract YAML resource builders from MigrationService into focused generator classes under service/generator/ (Gateway, HTTPRoute, AuthPolicy, RateLimitPolicy, PlanPolicy).

Acceptance criteria

• Generator classes own build logic; MigrationService delegates
• No behavior change; existing unit tests pass
• mvn test green

Part of architecture hardening phase 2 (task 2.2).