fix: restore coupon usage when invoices are cancelled

[my-dev-jour]

Summary

• add a Sales Invoice on_cancel hook to restore POS coupon usage after successful cancellation
• keep credit JE cleanup on before_cancel and realtime updates on on_cancel
• add regression tests for coupon rollback hook behavior

Testing

• python3 -m py_compile pos_next/api/sales_invoice_hooks.py pos_next/hooks.py pos_next/api/test_sales_invoice_hooks.py
• full Frappe unit tests were not run in this workspace because the frappe module is not installed here

Closes #195

[engahmed1190]

Code review

Found 1 issue:

1. rollback_coupon_usage has no is_return guard -- cancelling a return (credit note) invoice will decrement the coupon usage counter even though no usage was ever incremented for the return. Return invoices inherit coupon_code from the original invoice via ERPNext's make_sales_return. This can drive the used counter below its correct value, allowing single-use coupons to be reused. Add an early return for doc.get("is_return").

POSNext/pos_next/api/sales_invoice_hooks.py

Lines 158 to 167 in 1e9b0c3

	def rollback_coupon_usage(doc):
	"""Restore coupon usage for cancelled invoices that consumed a POS coupon."""
	coupon_code = doc.get("coupon_code")
	if not coupon_code or not frappe.db.table_exists("POS Coupon"):
	return
	try:
	from pos_next.pos_next.doctype.pos_coupon.pos_coupon import decrement_coupon_usage
	decrement_coupon_usage(coupon_code)

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[my-dev-jour]

Addressed. I added an early return for doc.get("is_return") in rollback_coupon_usage() so return/credit-note cancellations do not decrement coupon usage. I also added regression coverage for both the return-invoice path and the missing-coupon-table path.