Expose revocationDate and revocationReason on StoreTransaction (including iOS 26.4 RevocationType.proratedRefund)

[cruisediary]

Summary

SK2's Transaction exposes revocationDate and revocationReason since iOS 15,
but StoreTransaction does not surface these fields. iOS 26.4 also introduces
Transaction.RevocationType.proratedRefund for prorated refunds on subscription
downgrades.

Who needs this

This is not needed for standard RevenueCat usage — entitlement state is
already reflected in CustomerInfo via server-to-server notifications.

It is relevant for:

• CustomEntitlementComputation mode — entitlements are computed
  client-side, so revocation data must be read directly from the transaction
• Transaction history UI — displaying when/why a specific transaction
  was refunded
• Custom analytics — classifying refund events by reason in external pipelines

Proposed Changes

1. New RevocationReason type mirroring Transaction.RevocationType
   (following the existing TransactionReason pattern), with proratedRefund
   gated at @available(iOS 26.4, ...)
2. Add revocationDate: Date? and revocationReason: RevocationReason? to
   StoreTransactionType and expose via @objc public on StoreTransaction
3. SK1 returns nil for both fields (SK1 does not provide revocation data)
4. Update Swift and ObjC API testers

[facumenzella]

Thanks for the detailed explanation and proposal @cruisediary !! We'll take a look asap

[mvanhorn]

Implemented this in #6962.