fix(offers): preserve item discount on submit + harvest transaction-level rules

[engahmed1190]

Summary

Two related bugs surfaced from a real user report: a Sales Invoice ended up Partly Paid even though the cashier collected exactly the discounted amount the cart displayed. Diagnosis revealed two distinct issues in the offer engine that together cause every offered invoice to come out wrong.

Bug 1 — Item-level discount silently wiped on submit ("Partly Paid" bug)

When POS Next sets invoice.ignore_pricing_rule = 1 (which it always does, since offers are computed via apply_offers and POS Next handles discount fields manually) AND item.pricing_rules carries the offer name AND the doc already exists in DB, ERPNext's get_pricing_rule_for_item branches into remove_pricing_rule_for_item, which zeroes discount_percentage, discount_amount, and resets rate = price_list_rate on the next save.

• 1st save (update_invoice → draft): doc has no name yet → branch doesn't fire → discount survives ✓
• 2nd save (submit_invoice → reloads + saves submitted): doc exists → branch fires → discount silently zeroed ✗

Result: grand_total reverted to the pre-discount price; paid_amount stayed at the (discounted) amount the cashier collected; difference shows as outstanding_amount → status = Partly Paid.

Fix (pos_next/api/invoices.py:_process_invoice): clear item.pricing_rules before save. The discount is still preserved via discount_percentage / discount_amount / rate on the line — those fields are the source of truth, and the empty pricing_rules no longer triggers ERPNext's removal branch.

Bug 2 — Transaction-level Price discounts silently dropped

_evaluate_transaction_offers only harvested free-item rows after running ERPNext's apply_pricing_rule_on_transaction. When a Price-type Transaction rule fired (e.g., "10% off cart over 100 SAR"), ERPNext writes additional_discount_percentage / discount_amount / apply_discount_on on the doc header — and POS Next never read them. The cart received applied_pricing_rules: [] and never saw the discount.

Fix (pos_next/api/invoices.py:_evaluate_transaction_offers + apply_offers): snapshot the header fields before/after the engine call, surface them in the response dict, attribute the change to matching transaction-level Price rules so they appear in applied_pricing_rules. Frontend (POS/src/stores/posCart.js) writes the harvested discount_amount into additionalDiscount.value via a new applyHeaderDiscountFromServer helper. The existing submit path already maps additionalDiscount.value → invoice.discount_amount, so no other plumbing needed.

Test plan (automated)

Adds pos_next/test_promotions.py — 18 FrappeTestCase methods that drive the full apply_offers → update_invoice → submit_invoice pipeline and assert the saved invoice has the expected totals + status:

• 7 main shapes: Discount Percentage, Discount Amount, Rate override, Free same-item, Free different-item, Transaction-level, Mixed multi-item
• 6 edge cases: 100% off, discount = item price, multi-qty, stacked line + transaction, transaction below min_amt, plain no-offer invoice
• 2 regression canaries: test_partial_paid_regression + test_transaction_harvest_response pin down the exact behaviors fixed here
• 4 negative tests: disabled rule, expired rule, wrong item scope, transaction below min_amt — must yield empty applied_pricing_rules

The previously-commented Run Tests step in .github/workflows/ci.yml is now enabled and scoped to this module, so the suite gates every PR.

Local invocation on dev sites (where bench run-tests would wipe data):

bench --site <site> execute pos_next.test_promotions.run_all

5 consecutive local runs against nexus.local: 18/18 passing, deterministic.

Test plan (reviewer)

• Pull the branch and run bench --site <your-dev-site> execute pos_next.test_promotions.run_all; confirm 18/18 OK
• Reproduce the original "Partly Paid" bug on a stash of pre-fix code: apply a Discount Percentage Pricing Rule, add the item to cart, pay only the discounted amount → see Partly Paid
• Apply this branch, re-run the flow → status = Paid, outstanding_amount = 0
• Set up a Transaction-level rule (e.g. "10% off cart over 100 SAR"), check that the cart total reflects the discount and the saved invoice carries discount_amount / additional_discount_percentage / apply_discount_on = "Grand Total"
• Confirm CI green on this PR (the new Run Promotion Tests step)

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