validate_returned_items used a raw frappe.db.sql with a string-built column
list (and a separate Packed Item select); get_already_returned_items used a
raw GROUP BY sum. Convert both to frappe.get_all / frappe.qb (Sum(Abs(...))
with an explicit groupby). The qb GROUP BY mirrors the original
`group by item_code, <field>`, so it is parity-preserving (not a behaviour
change) and valid on Postgres.
Surgical re-apply: develop's `is_debit_note = 0` credit-note fix in
make_return_doc is preserved (the staging branch predated and would have
reverted it).
Adds a test (Delivery Note -> sales return) exercising validate_returned_items
and get_already_returned_items on both engines.
Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>
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