fix: incorrect rate in BOM exploded items

[s-aga-r]

Exploded Item rate showing incorrect value in the saved state.

https://user-images.githubusercontent.com/63660334/177125719-2cee7bc5-b6e3-4eca-801b-ceaaa0f17b48.mov

[s-aga-r]

Failing tests are not related #88e2b04

[marination]

@s-aga-r You can make the rate field read only and add a test to change qty in items table and check reflected changes in the exploded items table on save. 2 things to get this PR merged

[codecov[bot]]

Codecov Report

Merging #31513 (98e71fd) into develop (538cd6f) will increase coverage by 0.10%. The diff coverage is 100.00%.

@@             Coverage Diff             @@
##           develop   #31513      +/-   ##
===========================================
+ Coverage    63.10%   63.21%   +0.10%     
===========================================
  Files          813      813              
  Lines        57530    57530              
===========================================
+ Hits         36307    36366      +59     
+ Misses       21223    21164      -59     

Impacted Files	Coverage Δ
erpnext/manufacturing/doctype/bom/bom.py	87.95% <100.00%> (+0.15%)	:arrow_up:
.../report/delayed_item_report/delayed_item_report.py	60.78% <0.00%> (-35.30%)	:arrow_down:
...t/product_bundle_balance/product_bundle_balance.py	79.54% <0.00%> (-15.91%)	:arrow_down:
erpnext/accounts/party.py	77.43% <0.00%> (-4.46%)	:arrow_down:
erpnext/assets/doctype/asset/depreciation.py	85.23% <0.00%> (-2.69%)	:arrow_down:
erpnext/stock/report/stock_ledger/stock_ledger.py	75.79% <0.00%> (-2.55%)	:arrow_down:
...t/accounts/report/sales_register/sales_register.py	73.68% <0.00%> (-1.76%)	:arrow_down:
erpnext/crm/doctype/lead/lead.py	70.51% <0.00%> (-0.86%)	:arrow_down:
...ype/account/chart_of_accounts/chart_of_accounts.py	76.87% <0.00%> (-0.69%)	:arrow_down:
...xt/e_commerce/doctype/website_item/website_item.py	48.36% <0.00%> (-0.41%)	:arrow_down:
... and 20 more