fix: PayPal Express rule-based shipping methods not switched on country changed

[Chuc Le (untilu29)]

1. Why is this change necessary?

When the buyer changes the delivery country in the PayPal Express popup, the shipping callback switched the sales channel context to the new country but kept the previously selected shipping method ID. If that method was restricted by a country rule, the cart picked up a ShippingMethodBlockedError and the callback hard-failed with EXPRESS_SHIPPING_CALLBACK_METHOD_UNAVAILABLE - even when another shipping method was available for the new country

2. What does this change do, exactly?

After the context switch, if the cart now has a ShippingMethodBlockedError and ShippingOptionsProvider returns an available alternative, switch the context to the first available method and recalculate the cart so PayPal sees a valid selection

Also guard ExpressCustomerService::getAddressData against Shipping::$name being uninitialized — PayPal occasionally returns a Shipping struct without a name (e.g., in local-development mode), which caused a TypeError on the typed property access

3. Describe each step to reproduce the issue or behaviour.

4. Please link to the relevant issues (if any).

• relates PayPal Express: Rule based shipping methods don't work properly shopware#16295

5. Checklist

• I have written tests and verified that they fail without my change
• I have created an entry in the CHANGELOG.md files with all necessary user information about my changes
• This change has comments for package types, values, functions, and non-obvious lines of code
• I have read the contribution requirements and fulfill them.

[Max Stegmeyer (mstegmeyer)]

Not sure if we need this whole Shipping name fallback. AFAIK this was a Sandbox fluke a few days ago and not really something we should expect.