fix: correct previous monthly period for December [DHIS2-21584]

[eirikhaugstulen]

Summary

Fixes DHIS2-21584 by preventing getAdjacentFixedPeriods from returning the current monthly period when stepping backwards from December.

Root Cause

Monthly backward traversal used the period start date year and a slice boundary based on the first later period. For December, there is no later monthly period in the generated year, so the boundary calculation could include the current December period instead of ending before it.

Changes

• Use the year encoded in the period id when walking backwards through monthly periods.
• Treat the first current-or-following monthly period as the exclusive end boundary when collecting previous periods.
• Add a Gregorian regression test for stepping back from 202412 to 202411.

Validation

• yarn test get-adjacent-fixed-periods.gregorian.spec.ts --watchAll=false --runInBand --watchman=false
• yarn test get-adjacent-fixed-periods --watchAll=false --runInBand --watchman=false
• yarn d2-style check src/period-calculation/get-adjacent-fixed-periods/get-adjacent-fixed-periods.gregorian.spec.ts src/period-calculation/get-adjacent-fixed-periods/get-adjacent-monthly-fixed-periods.ts

[codecov-commenter]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 90.88%. Comparing base (d202f08) to head (aa5593a).

Additional details and impacted files

@@           Coverage Diff           @@
##             main      #96   +/-   ##
=======================================
  Coverage   90.87%   90.88%           
=======================================
  Files          57       57           
  Lines        1249     1250    +1     
  Branches      352      353    +1     
=======================================
+ Hits         1135     1136    +1     
  Misses        112      112           
  Partials        2        2           

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[sonarqubecloud]

Quality Gate passed

Issues
0 New issues
0 Accepted issues

Measures
0 Security Hotspots
0.0% Coverage on New Code
0.0% Duplication on New Code

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