(feat) O3-5413: Add REFUND_REQUESTED and REFUNDED bill statuses

[UjjawalPrabhat]

Summary

This PR adds backend support for the frontend to enable refund requests and approval for paid bills.

• Add REFUND_REQUESTED and REFUNDED to BillStatus enum
• Allow PAID → REFUND_REQUESTED and REFUND_REQUESTED → REFUNDED status transitions, enforcing REFUND_MONEY privilege for issuing refunds
• Guard synchronizeBillStatus() from overwriting refund statuses during payment recalculation

Related Issue

O3-5413

[UjjawalPrabhat]

@NethmiRodrigo @wikumChamith @ibacher Could you please review this PR?

[ELVIS-KATO]

@UjjawalPrabhat thank you for the work done, the changes LGTM, but if you don't mind, For when rejecting a refund request (REFUND_REQUESTED → PAID), consider clearing the refund request metadata for clarity. (Optional but could make the system cleaner when cleared)

[NethmiRodrigo]

Thanks @UjjawalPrabhat, but can we have a separate endpoint to handle refunds ?

[NethmiRodrigo]

consider clearing the refund request metadata for clarity. (Optional but could make the system cleaner when cleared)

The whole point of recording that metadata is to have this record for reporting or auditing purposes so clearing it would be bad

[codecov-commenter]

Codecov Report

❌ Patch coverage is 10.00000% with 45 lines in your changes missing coverage. Please review.
✅ Project coverage is 26.51%. Comparing base (1778231) to head (1a04d9e).

Files with missing lines	Patch %	Lines
...module/billing/web/rest/resource/BillResource.java	0.00%	37 Missing ⚠️
...ling/web/rest/controller/BillRefundController.java	0.00%	8 Missing ⚠️

Additional details and impacted files

@@             Coverage Diff              @@
##               main     #108      +/-   ##
============================================
- Coverage     26.73%   26.51%   -0.22%     
- Complexity      487      489       +2     
============================================
  Files           190      191       +1     
  Lines          4309     4359      +50     
  Branches        486      493       +7     
============================================
+ Hits           1152     1156       +4     
- Misses         3056     3102      +46     
  Partials        101      101              

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