fix: incorrect net profit in Profit and Loss Statement

[ruthra-kumar]

When Accumulated Values filter is enabled, P&L reported incorrect Net Profit amount. When it's enabled, each period has a running balance in the report. So, picking the last period's balance should be enough, rather than summing all of them.

Todo:

• add test case

before

after

[codecov]

Codecov Report

Attention: 6 lines in your changes are missing coverage. Please review.

Comparison is base (a09241e) 60.19% compared to head (d54f831) 60.20%.
Report is 86 commits behind head on develop.

❗ Current head d54f831 differs from pull request most recent head 8555617. Consider uploading reports for the commit 8555617 to get more accurate results

Additional details and impacted files

@@             Coverage Diff             @@
##           develop   #38969      +/-   ##
===========================================
+ Coverage    60.19%   60.20%   +0.01%     
===========================================
  Files          759      759              
  Lines        69767    69921     +154     
===========================================
+ Hits         41995    42098     +103     
- Misses       27772    27823      +51     

Files	Coverage Δ
.../report/accounts_receivable/accounts_receivable.py	89.73% <100.00%> (+0.05%)	⬆️
erpnext/controllers/item_variant.py	68.62% <100.00%> (-2.73%)	⬇️
erpnext/controllers/selling_controller.py	79.94% <100.00%> (ø)
...cturing/doctype/production_plan/production_plan.py	80.75% <100.00%> (+0.58%)	⬆️
...stock/doctype/material_request/material_request.py	69.67% <100.00%> (ø)
...wise_balance_history/batch_wise_balance_history.py	74.69% <100.00%> (+0.94%)	⬆️
erpnext/accounts/report/financial_statements.py	80.00% <71.42%> (-0.50%)	⬇️
...it_and_loss_statement/profit_and_loss_statement.py	69.41% <69.23%> (-5.92%)	⬇️

... and 22 files with indirect coverage changes

[ruthra-kumar]

ignoring unrelated test failure: https://github.com/frappe/erpnext/actions/runs/7346736657/job/20001985082?pr=38969