fix: rewrite logic for duplicate check in Item Attribute

[SvbZ3r0]

Previously, Item Attribute values were not checked for case-insensitive duplicates, and Item tttribute abbreviations were forced to be uppercase. This commit fixes both problems.

Fixes: #22256
Partially fixes: #15467

[s-aga-r]

@SvbZ3r0 please fix the failing test.

[SvbZ3r0]

I've fixed the formatting, but I'm not sure what the other failing tests have to do with any changes I made here.

[codecov]

Codecov Report

Merging #33619 (bfbc9ea) into develop (2c83fff) will increase coverage by 0.00%.
The diff coverage is 50.00%.

❗ Current head bfbc9ea differs from pull request most recent head 333b2ae. Consider uploading reports for the commit 333b2ae to get more accurate results

Additional details and impacted files

@@           Coverage Diff            @@
##           develop   #33619   +/-   ##
========================================
  Coverage    64.14%   64.14%           
========================================
  Files          819      819           
  Lines        59165    59093   -72     
========================================
- Hits         37950    37906   -44     
+ Misses       21215    21187   -28     

Impacted Files	Coverage Δ
...ext/stock/doctype/item_attribute/item_attribute.py	88.09% <50.00%> (-0.28%)	⬇️
erpnext/stock/doctype/pick_list/pick_list.py	76.59% <0.00%> (-1.36%)	⬇️
erpnext/stock/doctype/serial_no/serial_no.py	89.20% <0.00%> (-0.24%)	⬇️
erpnext/stock/get_item_details.py	80.61% <0.00%> (-0.18%)	⬇️
...e/payment_reconciliation/payment_reconciliation.py	96.99% <0.00%> (-0.02%)	⬇️
erpnext/controllers/taxes_and_totals.py	95.15% <0.00%> (-0.01%)	⬇️
...rpnext/assets/doctype/asset_repair/asset_repair.py	66.04% <0.00%> (ø)
.../report/tds_payable_monthly/tds_payable_monthly.py	0.00% <0.00%> (ø)
...ype/repost_payment_ledger/repost_payment_ledger.py	0.00% <0.00%> (ø)
...x_withholding_category/tax_withholding_category.py	85.90% <0.00%> (ø)
... and 8 more

[SvbZ3r0]

@s-aga-r This is passing all tests. Let me know if I need to change anything else.