feat: a BOM cost update should re-calculate operating costs off a workstation vs using cached hour_rate field

[HKuz]

This issue came up working on a client feature that depends on the Workstation Operating Cost feature. Currently, Inventory Tools will update the hour_rate field on Workstation in before_save, grabbing the appropriate current-date-based costs from the Workstation Operating Cost table and setting the field, with a fallback to ERPNext's calc to use the component fields (labor + electricity + etc).

The client's use-case is they will add new Workstation Operating Cost rows with valid_from dates happening in the future, evaluate costing, then trigger a 'rollover' (happens on the from_date), which also updates BOM costs. This bypasses the before_save hook, and ERPNext's BOM cost calcs grab a cached value from the hour_rate field on Workstation, which will be the stale rates.

Proposed fix:

• Create an InventoryToolsBOM class
• Create override in UI for when a row is added to the BOM Operation table, the row's hour_rate should check the Workstation Op Cost table dates against current date (or if no Workstation Op Cost rows, uses fallback to sum of static field values for labor/electricity/etc)
• Create override on InventoryToolsBOM class for update_rate_and_time when costs are updated
• If the rate changed in either case, should set the Workstation's hour_rate field to the new value in the DB