Post-hardening C2: extract BackendEndpointResolver from MigrationService

[fmenesesg]

Summary

Wave C PR2: Extract 3scale backend endpoint resolution from MigrationService into service/migration/BackendEndpointResolver.

Scope

• BackendEndpointResolver with BackendIndex, resolveIndex(), endpoint parsing helpers
• MigrationService delegates backend lookup; drops unused ThreeScaleSourceRegistry field
• Unit tests for index building and DNS parsing

Acceptance criteria

• No resolveBackendEndpoints / BackendIndex in MigrationService
• BackendEndpointResolverTest passes
• mvn test passes