refactor: move item_code reactivity to server-side

[ruthra-kumar]

Context

Selecting an Item is the most common action in Sales / Purchase cycle. Whenever an Item is selected, system fetches the basic details like rate, discounts, stock qty, warehouse and sales account as such and auto-fills these details in it's respective fields. This behavior can simply be called as Reactivity.

It is achieved by a JS trigger on item_code located in transaction.js

erpnext/erpnext/public/js/controllers/transaction.js

Line 499 in 5266f23

item_code(doc, cdt, cdn) {

But, due to a lack of UI test cases, this Reactivity is prone to regression, especially the ones related to discounts and free items. Check #42195 were the broken scenarios are individually identified.

Long-Term Stability and Maintenance

To prevent frequent regression, we've decided to move most of the Reactivity to the server-side, as it allows for server-side tests and a single source to maintain.

This PR is the first in that direction. It moves all of the UI Reactivity associated with Item selection server-side.

Impacts

Purchase Invoice      
POS Invoice           
Sales Invoice         
Purchase Order        
Request for Quotation 
Supplier Quotation    
Quotation             
Sales Order           
Purchase Receipt