fix: preserve UOM conversion factor precision in transactions

calculate_item_values rounds every Float field on an item row to the
site's Float Precision (3 by default), and conversion_factor was one of
them. The factor is a ratio, not a rate: UOM Conversion Factor.value is
stored at precision 9, and Material Request keeps the full value because
it has no currency field and so never runs the calculation.

Mapping a Material Request to a Purchase Order therefore truncated the
factor - 0.453592292 for Pound -> Kg became 0.454 - and stock_qty, which
is recomputed as qty * conversion_factor, drifted from the quantity that
was requested, leaving the Material Request unable to close.

Exclude conversion_factor from the rounded fields on the server and on
the client. Factors below the site precision would otherwise round to
zero outright.

erpnext/controllers/buying_controller.py

@@ -466,7 +466,7 @@ class BuyingController(SubcontractingController):
 					self.precision("item_tax_amount", item),
 				)
 
-				self.round_floats_in(item)
+				self.round_floats_in(item, do_not_round_fields=["conversion_factor"])
 				if flt(item.conversion_factor) == 0.0:
 					item.conversion_factor = (
 						get_conversion_factor(item.item_code, item.uom).get("conversion_factor") or 1.0

erpnext/controllers/taxes_and_totals.py

@@ -225,7 +225,12 @@ class calculate_taxes_and_totals:
 		if self.doc.get("is_consolidated") or self.discount_amount_applied:
 			return
 
-		do_not_round_fields = ["valuation_rate", "incoming_rate", "sales_incoming_rate"]
+		do_not_round_fields = [
+			"valuation_rate",
+			"incoming_rate",
+			"sales_incoming_rate",
+			"conversion_factor",
+		]
 		for item in self.doc.items:
 			self.doc.round_floats_in(item, do_not_round_fields=do_not_round_fields)
 			self.calculate_item_rate(item)