order.Info.Receipt Payload always empty in IAP 5.1.0+ due to SetRefreshAppReceipt default change

Description
In IAP 5.1.0, the default value of m_RefreshAppReceipt in AppleStoreImpl was silently changed from true to false as part of as part of the StoreKit 1 reinstatement work. This causes order.Info.Receipt to always return an empty Payload field for StoreKit 2 purchases, breaking any third-party SDK or backend that relies on the App Receipt for iOS purchase validation.

The issue has two compounding problems:

1. The XML documentation on IAppleStoreExtendedPurchaseService.SetRefreshAppReceipt still states "True by default", which is factually incorrect and actively misleads developers debugging the problem.
2. SetRefreshAppReceipt is marked [Obsolete], meaning the only available workaround uses a deprecated API. There is no non-deprecated path to populate the App Receipt in IAP 5.x for customers still using traditional receipt validation workflows.

This was reported by a customer who experienced a 25-30% drop in validated revenue events after upgrading from IAP 5.0.4 to 5.2.1. All purchases reported to Singular were marked as {}iapInvalid{} due to the empty Payload.

Initial setup

• IAP version: 5.2.1
• Unity Editor version: unknown (customer-reported)
• Platform(s): iOS
• StoreKit version: StoreKit 2
• Git branch: develop (regression introduced in commit 040a13cc9, released in 5.1.0)

Reproduction steps

• Upgrade a project from IAP 5.0.4 to IAP 5.1.0 or later
• Connect to the Apple App Store using the new 5.x API (StoreController)
• Subscribe to OnPurchasePending
• Complete a purchase on a physical iOS device running iOS 15 or later (StoreKit 2 path)
• In the OnPurchasePending handler, log order.Info.Receipt
• Observe that the Payload field is empty

Expected behaviour
order.Info.Receipt should contain a populated Payload field containing the base64-encoded App Receipt, consistent with IAP 5.0.x behaviour and consistent with the existing XML documentation which states the refresh is "True by default".

{"Payload":"MIIUZ...==","Store":"AppleAppStore","TransactionID":"2000001162882605"}

Actual behavior
order.Info.Receipt always returns an empty Payload because m_RefreshAppReceipt defaults to false in AppleStoreImpl.cs. This affects both OnPurchasePending (new purchases) and OnPurchasesFetched (fetched confirmed orders).

{"Payload":"","Store":"AppleAppStore","TransactionID":"2000001164329034"}

The only available workaround is calling the deprecated SetRefreshAppReceipt(true):

Unable to find source-code formatter for language: csharp. Available languages are: actionscript, ada, applescript, bash, c, c#, c++, cpp, css, erlang, go, groovy, haskell, html, java, javascript, js, json, lua, none, nyan, objc, perl, php, python, r, rainbow, ruby, scala, sh, sql, swift, visualbasic, xml, yamlstoreController.AppleStoreExtendedPurchaseService?.SetRefreshAppReceipt(true);

The XML documentation on IAppleStoreExtendedPurchaseService is also incorrect:

// TODO: IAP-3929
/// [summary]
/// Indicates whether to automatically refresh the app receipt when a purchase is made.
/// This is useful if you are still using the receipt to validate purchases.
/// True by default.        [-- INCORRECT: default is false since ULO-7255
/// </summary]
[Obsolete]
void SetRefreshAppReceipt(bool refreshAppReceipt);