fix: repost read stale sibling SLE rate for moving average returns

During repost, a return line with recalculate_rate resolved its moving
average rate through get_incoming_rate -> get_previous_sle, which matches
posting_datetime <= and orders by creation desc. For a multi-line return
of the same item, every line shares one posting_datetime, so the query
landed on a sibling line of the same voucher whose stored valuation_rate
was still the previous repost run's output, not the rate before the
voucher.

Each repost run therefore re-seeded the voucher from its own prior
output. The error gain per run is (qty returned at the stale rate) /
(qty remaining after the return), so whenever a return removes most of
the stock the loop diverges instead of converging, alternating sign and
growing until stock_value overflows decimal(21,9) and the repost dies
with 'Out of range value for column stock_value'.

Use the in-memory running valuation rate that update_entries_after
already tracks for the warehouse at this point in the repost. It is the
authoritative pre-entry state, is immune to sibling rows, and makes the
repost idempotent. The database lookup is kept only as a fallback for a
zero in-memory rate, preserving the existing zero-rate fallback chain.

erpnext/stock/stock_ledger.py

@@ -1301,6 +1301,8 @@ class update_entries_after:
 					and not sle.get("batch_no")
 					and not sle.get("serial_and_batch_bundle")
 				):
+					rate = flt(self.wh_data.valuation_rate)
+					if not rate:
 						rate = get_incoming_rate(
 							{
 								"item_code": sle.item_code,