refactor: stock balance report

[ankush]

• set up tests
• query conversions, remove all raw SQL.
• cover most used scenarios in tests
• minor code cleanup here and there.

don't backport

[ankush]

Each warehouse having roughly 1000 unique item and 6000-10000 SLEs
Single warehouse:
Before: 50ns
After: 32ns

Group of 5 warehouse:
Before: 57ns
After: 35ns

A very marginal difference which is eliminated once # of child warehouses ~15-20 and will be somewhat slower on numbers higher than that. Not sure if this is worth it. The main difference will likely come from rewriting the whole qty and value logic in SQL.

[codecov]

Codecov Report

Merging #30677 (7448d7e) into develop (1659f1e) will decrease coverage by 0.04%.
The diff coverage is 96.96%.

❗ Current head 7448d7e differs from pull request most recent head 9af2d68. Consider uploading reports for the commit 9af2d68 to get more accurate results

@@             Coverage Diff             @@
##           develop   #30677      +/-   ##
===========================================
- Coverage    61.43%   61.39%   -0.05%     
===========================================
  Files         1080     1079       -1     
  Lines        69488    69265     -223     
===========================================
- Hits         42693    42523     -170     
+ Misses       26795    26742      -53     

Impacted Files	Coverage Δ
...rpnext/stock/report/stock_balance/stock_balance.py	88.58% <96.96%> (+10.48%)	⬆️
...payroll/doctype/income_tax_slab/income_tax_slab.py	83.33% <0.00%> (-16.67%)	⬇️
erpnext/accounts/doctype/bank/bank.py	71.42% <0.00%> (-14.29%)	⬇️
...rial_no_valuation/incorrect_serial_no_valuation.py	85.96% <0.00%> (-10.53%)	⬇️
...t/accounts/report/sales_register/sales_register.py	70.76% <0.00%> (-7.02%)	⬇️
...pnext/accounts/report/gross_profit/gross_profit.py	81.48% <0.00%> (-5.77%)	⬇️
...ctype/woocommerce_settings/woocommerce_settings.py	80.76% <0.00%> (-3.85%)	⬇️
erpnext/hr/doctype/job_applicant/job_applicant.py	63.93% <0.00%> (-3.28%)	⬇️
erpnext/education/doctype/student/student.py	73.68% <0.00%> (-3.16%)	⬇️
...next/accounts/doctype/bank_account/bank_account.py	77.27% <0.00%> (-2.28%)	⬇️
... and 33 more