Fix missing onCloseHandler in CustomerCenterProxy

[facumenzella]

Summary

• CustomerCenterProxy.createCustomerCenterViewController() never set onCloseHandler on the CustomerCenterUIViewController it created
• When onCloseHandler is nil, CustomerCenterView falls back to SwiftUI's @Environment(\.dismiss) action
• On iOS 15, calling dismiss() at the root of a NavigationView is a no-op (there's even a comment in DismissCircleButton calling this out explicitly)
• This meant the X close button had no effect when the Customer Center was presented via presentCustomerCenter() on iOS 15

The fix sets onCloseHandler to call vc.dismiss(animated: true), which:

1. Properly dismisses via UIKit on all iOS versions
2. Triggers viewDidDisappear with isBeingDismissed = true
3. Calls customerCenterViewControllerWasDismissed on the delegate → resolves the RN/Flutter/etc. Promise

Note: the embedded CustomerCenterViewManager already set onCloseHandler correctly — this only affected the modal proxy path.

Test plan

• Present Customer Center via presentCustomerCenter() on iOS 15 device/simulator
• Navigate to "Display Purchase History"
• Tap the X close button — should dismiss the entire Customer Center
• Confirm the Promise resolves and onDismiss fires

Fixes CC-647

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