Reshape Checkout Session tax

[yuki-stripe]

Summary

Match the reviewed Checkout Elements API by separating tax computation state from aggregate tax amounts.

The Checkout Playground continues to use totals for its compact order summary and exposes the named taxAmounts breakdown from an info button in both UIKit and SwiftUI.

Before

Session
`-- tax: Checkout.Tax
    +-- status
    `-- taxAmounts?

After

Session
+-- tax: Session.Tax?
|   `-- status
`-- taxAmounts: [Session.TaxAmount]?

taxAmounts describes the individual aggregate tax lines, while totals groups their values by tax treatment:

taxAmounts
+-- State Tax:  $5 exclusive
+-- County Tax: $2 exclusive
`-- VAT:        $1 inclusive

totals.taxExclusive = $7
totals.taxInclusive = $1

Tax computation state maps as follows:

automatic + complete                   -> Tax(.ready)
automatic + requires shipping address  -> Tax(.requiresShippingAddress)
automatic + requires billing address   -> Tax(.requiresBillingAddress)
missing, failed, or unsupported         -> nil
non-automatic tax                       -> Tax(.ready)

tax == nil means that no valid public computation status is available. It does not mean that no tax applies or that tax computation succeeded.

Testing

CleanShot.2026-08-14.at.17.06.48.mp4

Added test coverage confirming:

• Aggregate tax amounts preserve their formatted amount, minor-unit amount, inclusive flag, display name, and percentage.
• An absent aggregate tax amount list remains nil, while an explicitly empty list remains empty.
• Completed automatic tax maps to the ready state.
• Automatic tax that needs an address requests the configured shipping or billing address.
• Failed, unsupported, or incomplete automatic-tax metadata produces no public tax state.
• Non-automatic tax maps to the ready state.

Manually verified the Checkout Playground tax summary and tax-details presentation in both UIKit and SwiftUI.

Stack

Depends on the Checkout Session status PR (yuki/checkout-session-status).