[in_app_purchase_storekit] Add show manage subscriptions path

[Hari-07]

Adds a new showManageSubscriptions method which wraps around storekit's showManageSubscriptions(in:)

Fixes flutter/flutter#159664

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[gemini-code-assist]

Code Review

This pull request introduces the showManageSubscriptions method to the InAppPurchaseStoreKitPlatformAddition class, enabling users to manage subscriptions via the App Store sheet on iOS 15+ using StoreKit 2. The changes include the Swift implementation, Pigeon-generated code, an example app button, and corresponding unit tests. Feedback recommends moving UIKit property access inside the @MainActor task block in Swift to ensure thread safety, and catching potential exceptions in the example app's onPressed callback to prevent unhandled asynchronous errors.

[gemini-code-assist]

Accessing UIKit properties such as viewController, view, window, and windowScene must be done on the main thread (@MainActor). Currently, the guard statement retrieves the windowScene before entering the @MainActor task block, which can lead to thread-safety issues or crashes if this method is invoked from a background thread. Moving the entire guard statement inside the @MainActor task ensures safe UI-related operations.

Suggested change

	guard let windowScene = self.registrar?.viewController?.view.window?.windowScene else {
	let error = PigeonError(
	code: "storekit2_missing_key_window_scene",
	message: "Failed to fetch key window scene",
	details: "registrar.viewController.view.window.windowScene returned nil."
	)
	completion(.failure(error))
	return
	}
	Task { @MainActor in
	do {
	try await AppStore.showManageSubscriptions(in: windowScene)
	completion(.success(()))
	} catch {
	completion(.failure(error))
	}
	}
	Task { @MainActor in
	guard let windowScene = self.registrar?.viewController?.view.window?.windowScene else {
	let error = PigeonError(
	code: "storekit2_missing_key_window_scene",
	message: "Failed to fetch key window scene",
	details: "registrar.viewController.view.window.windowScene returned nil."
	)
	completion(.failure(error))
	return
	}
	do {
	try await AppStore.showManageSubscriptions(in: windowScene)
	completion(.success(()))
	} catch {
	completion(.failure(error))
	}
	}

[Hari-07]

I think this is more correct but I was reflecting the pattern in presentOfferCodeRedeemSheet. Happy to update this one or both

[gemini-code-assist]

The showManageSubscriptions method returns a Future<void> and can throw an exception (for example, on unsupported platforms like macOS or iOS versions below 15). Since this future is not awaited or caught in the onPressed callback, any thrown error will result in an unhandled asynchronous exception. It is highly recommended to catch the error and display a user-friendly message (e.g., using a SnackBar) to handle these expected platform limitations gracefully.

            onPressed: () async {
              try {
                await _iapStoreKitPlatformAddition.showManageSubscriptions();
              } catch (e) {
                if (mounted) {
                  ScaffoldMessenger.of(context).showSnackBar(
                    SnackBar(content: Text('Error: $e')),
                  );
                }
              }
            },

[Hari-07]

I think this is ok for the internal example

[Hari-07]

I wasn't sure if we need to do something in in_app_purchase. From what I can see the code change will get picked up automatically maybe just need to update the readme. And bump dependency to ^0.4.12 so that the readme is correct, if we want to