magento/magento2#32468: Fix cart-fixed discount missing shipping VAT

[swnsma]

Fixed Issues

1. Fixes Cart Price Rules not applying on shipping VAT when rule applied on whole cart #32468 — Cart price rules not applying on shipping VAT when rule applied on whole cart

Manual testing scenarios

1. Configure tax: shipping tax class taxable, catalog/shipping/apply discount on prices Including Tax, cross-border trade Yes, flat rate per order ($5).
2. Create cart price rule: fixed amount for whole cart 56, apply to shipping Yes, coupon.
3. Add product priced 51 (incl tax class), apply coupon and set flat rate shipping.
4. Expected: discount −56, shipping discount 5.00 (not 4.17 excl), grand total 0.
5. Verify coupon-before-shipping and shipping-before-coupon both recollect correctly.
6. Multi-shipping cart-fixed paths are intentionally unchanged (existing multishipping cart-fixed tests remain the regression lock).

Questions or comments

Single-shipping cart-fixed now seeds remaining balance after item allocation and caps shipping discount using the tax discount basis (ShippingAmountForDiscount when set). Multi-shipping keeps the previous dual-gate detection and proportional formula to minimize regression surface.

Includes unit tests and integration suite CartFixedShippingTaxTest for the ticket matrix, partial remaining, discount-never-exceeds-rule, and related tax configs.

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[swnsma]

@magento run all tests

[swnsma]

@magento run Functional Tests B2B,Functional Tests CE,Functional Tests EE,
WebAPI Tests