feat(payment): PAYPAL-6433 implement server side shipping callbacks logic

[andriiVitvitskyi1990]

What/Why?

Implemented server side shipping callbacks logic

Rollout/Rollback

Revert PR

Testing

Unit tests

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Note

Medium Risk
Changes PayPal hosted-checkout approval and updateOrder behavior around shipping data, which can affect address/shipping selection and order submission for physical carts. Also removes AppSwitch resume/appSwitchWhenAvailable handling, so regressions could impact mobile/app-return flows.

Overview
Implements a new isServerSideShippingCallbacksEnabled initialization flag across PayPal Commerce (PayPal + Credit/PayLater) strategies and shared utils to toggle client vs server handling of shipping callbacks.

When enabled, PayPal button configs omit onShippingAddressChange/onShippingOptionsChange, approval flows avoid pulling actions.order.get() for addresses, updateOrder sends no shipping options ([]) and selectedShippingOption: null, and checkout is reloaded before order submission for physical-item carts.

Separately, AppSwitch support is removed from these PayPal strategies/utilities (no appSwitchWhenAvailable, hasReturned()/resume() paths), with unit tests updated to reflect the new flag-driven behavior.

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