FINERACT-2651: Refactor MixReportApiResource to return type-safe JAX-RS Response

[DhroovSankla]

Description

This pull request addresses architectural refactoring for the Mix Report module by enforcing JAX-RS framework best practices and cleaning up obsolete placeholders.

Proposed Changes

1. Type Safety Implementation: Refactored retrieveXBRLReport inside MixReportApiResource.java to return an explicitly typed jakarta.ws.rs.core.Response wrapper entity instead of a raw String. This ensures proper JAX-RS native response handling and standard content negotiation.
2. Code Cleanup: Removed outdated structural TODO comments regarding marshalling engines and type safety from MixReportApiResource.java and MixReportXBRLBuilder.java.

Related Issue

Fixes: FINERACT-2651

Checklist

• My code follows the code style of this project.
• All new and existing tests passed (handled via automated CI pipeline).
• Commits are signed using GPG keys.