Improve backend transform output consistency

[maxx1128]

Issue Description

For the backend transform files in modules/debts_api/lib/debts_api/v0/fsr_form_transform, there's some inconsistency in the output hash objects that make the classes less readable and are a minor code smell. Some output a full hash object, such as gmt_calculator#income_limits. Others start with an empty one and build on it, such as asset_calculator#transform_assets. Virtually all cases could go with the former.

Back end tasks

• [x] Check all transform classes outputting hashes and update them to returning a full hash object instead of building on an empty one (unless it's specifically unavoidable in those cases)

Acceptance Criteria

• [ ] All specs still pass after refactors are made.

Appendix

Product notes

Tech notes

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[maxx1128]

As of the end of Sprint 129, this ticket's pull request needs to be approved before it is ready to merge. However, we're concerned that merging it will negatively affect testing. So, we're holding off on the final merge even after it is approved.