Fix after_cashback_amount_total to use the line total

[josecarneiro]

In applyDiscounts, the cashback branch computed after_cashback_amount_total from the per-unit gross amount instead of the line total:

afterCashbackAmountTotal = unitAmountGross.subtract(normalizedCashbackAmount);

When the quantity multiplier is 1, unit_amount_gross === amount_total, so the result was correct by coincidence. But when the unit amount is multiplied by a quantity/consumption (so amount_total = unit_amount_gross × multiplier), subtracting the full cashback from the small per-unit figure produced wrong — often negative — totals. This also left the item-level value inconsistent with the recurrence-level after_cashback_amount_total, which was already derived from the total.

Fix

Deduct the cashback from amount_total. Item-level and recurrence-level values now reconcile (the item afters sum to the recurrence after).

• apply-discounts.ts — use amount_total as the base.
• pricing.results.ts — corrected a fixture that encoded the old value (8000 → 18000; amount_total 20000 − cashback 2000).
• pricing-getag.test.ts — added regression coverage for the multiplier-≠-1 cashback case, plus a withCouponOnComponents helper.

Only after_cashback_amount_total changes, and only when the multiplier ≠ 1; it's a leaf output, so aggregates are unaffected.

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