fix: incorrect bin qty on backdated reconciliation

[ankush]

Bin quantities are updated by adding actual_qty from current SLE to
Bin's qty. In stock reconciliation qty is "reset" hence
qty_after_transaction is used to set qty in bin.

However, in the case of backdated stock reconciliation, there are future
transactions that will affect the final sum of quantity available in bin.

Note: This gets auto-corrected after reposting finishes for backdated
transactions, but Bin shows wrong quantity till then. Bin is used in item
dashboard hence it's confusing. Also, reposting is only supposed to be
for valuation, qty should be correct in real-time.

Solution: Instead of overriding Bin's quantity, replace it with last sle qty in case of backdated transactions.

To test:

1. Pick one item - warehouse combination.
2. Create some stock transactions.
3. Open Bin and verify that current quantity is correct.
4. Create a backdated stock reconciliation.
5. Confirm that Bin's quantity matches with stock ledger and stock
   balance. (earlier this used to get reset to whatever stock reco
   quantity was)
6. Cancel the stock reconciliation and verify stock ledger/balance
   again.

Example transactions ordered by creation; more test cases on #26158

Transaction	Posting date	Qty	qty in Bin
Create A	0	+1	1
Create B	-1	2 (reset via stock reco)	3 (this used to be 2 before)
Cancel B	-1	0	1 (this used to be 0 before)

[codecov]

Codecov Report

Merging #28588 (58cc6d3) into develop (66a10c0) will increase coverage by 0.04%.
The diff coverage is 85.71%.

❗ Current head 58cc6d3 differs from pull request most recent head f58728a. Consider uploading reports for the commit f58728a to get more accurate results

@@             Coverage Diff             @@
##           develop   #28588      +/-   ##
===========================================
+ Coverage    55.59%   55.64%   +0.04%     
===========================================
  Files         1128     1128              
  Lines        67285    67286       +1     
===========================================
+ Hits         37404    37438      +34     
+ Misses       29881    29848      -33     

Impacted Files	Coverage Δ
erpnext/stock/doctype/bin/bin.py	87.50% <83.33%> (+0.22%)	⬆️
erpnext/stock/stock_ledger.py	85.39% <100.00%> (+0.36%)	⬆️
...payroll/doctype/income_tax_slab/income_tax_slab.py	83.33% <0.00%> (-16.67%)	⬇️
.../hr/doctype/upload_attendance/upload_attendance.py	44.69% <0.00%> (-1.52%)	⬇️
.../report/stock_projected_qty/stock_projected_qty.py	88.15% <0.00%> (-1.32%)	⬇️
erpnext/support/doctype/issue/issue.py	64.03% <0.00%> (-0.99%)	⬇️
erpnext/stock/report/stock_ledger/stock_ledger.py	77.23% <0.00%> (-0.82%)	⬇️
erpnext/selling/doctype/sales_order/sales_order.py	77.58% <0.00%> (-0.39%)	⬇️
erpnext/stock/get_item_details.py	79.29% <0.00%> (-0.16%)	⬇️
...stock/doctype/purchase_receipt/purchase_receipt.py	90.64% <0.00%> (+0.26%)	⬆️
... and 10 more

[ankush]

this (and all other cases of bin update) are broken in the case of intermediate stock reconciliation. refer: #26158

[ankush]

uncovered lines intentionally don't run in the tests. coverage is 100% actually 😄