fix: always set correct gst treatment (backport #3751)

[mergify]

Issue: incorrect gst treatment if the item tax template is changed from the backend or the item tax template is updated from exempted to taxable.

Steps to replicate:

• Set the item tax template in the item as 18%.
• Create a Sales Invoice and save it.
• Item tax template will be 18% and gst treatment will be taxable.
• Change the item tax template in Item to Exempted
• Again, save the invoice.

GST treatment will be Taxable, but the Item Tax Template will be Exempted because erpnext changed it.

Summary by CodeRabbit

• Refactor
  • Optimized GST treatment mapping logic for improved transaction processing efficiency.

---

This is an automatic backport of pull request #3751 done by [Mergify](https://mergify.com).

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[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 59.89%. Comparing base (ed09675) to head (f189648).
⚠️ Report is 2 commits behind head on version-15-hotfix.

Additional details and impacted files

@@                  Coverage Diff                  @@
##           version-15-hotfix    #3753      +/-   ##
=====================================================
- Coverage              59.91%   59.89%   -0.02%     
=====================================================
  Files                    140      140              
  Lines                  14799    14792       -7     
=====================================================
- Hits                    8867     8860       -7     
  Misses                  5932     5932              

Files with missing lines	Coverage Δ
...ndia_compliance/gst_india/overrides/transaction.py	83.63% <100.00%> (-0.13%)	⬇️

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[india-compliance-bot]

🎉 This PR is included in version 15.23.1 🎉

The release is available on GitHub release

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