Extract BillingAddressElement configuration

[cttsai-stripe]

Summary

Extracts BillingAddressElement.Configuration, an immutable value holder for the static inputs used to construct a billing-address element.

• Migrate the card, standalone billing-details, and CountrySpec callers without changing their inputs.
• Keep DropdownFieldController and SameAsShippingElement as constructor inputs because they own live form state.
• Leave automatic-tax policy and field widening out of this change.

Motivation

#13713 needs to replace a billing-address element when automatic tax widens a country-only form. A copyable configuration makes that replacement explicit without mutating an element whose nested address and section controllers have already captured live state.

BillingAddressElement(
    configuration = BillingAddressElement.Configuration(/* static construction policy */),
    countryDropdownFieldController = /* live country state */,
    sameAsShippingElement = /* live checkbox state */,
)

The CountrySpec path continues to initialize its controller from initialValues[spec.apiPath]. Card and standalone billing-details paths continue to use IdentifierSpec.Country.

This is behavior-neutral prework between #13737 and #13713. It intentionally does not add withAdditionalFields(), automatic-tax constants, gates, or finalization.

Path	Approve only if	Evidence
CountrySpec	The custom API path and initial country remain on the same controller.	TransformSpecToElementsTest
Card	The country controller uses the allowed countries and IdentifierSpec.Country initial value.	CardDefinitionTest
Standalone billing details	Initial values and Automatic/Never visibility stay unchanged.	BillingDetailsFormTest
Scope	No finalizer, widening method, or automatic-tax decision is introduced.	Diff review

Testing

• Added tests
• Modified tests
• Manually verified

Passed:

• ./gradlew :payments-ui-core:testDebugUnitTest --tests com.stripe.android.ui.core.elements.CardBillingAddressElementTest
• ./gradlew :paymentsheet:testDebugUnitTest --tests com.stripe.android.lpmfoundations.luxe.TransformSpecToElementsTest --tests com.stripe.android.lpmfoundations.paymentmethod.definitions.CardDefinitionTest --tests com.stripe.android.paymentsheet.ui.BillingDetailsFormTest
• ./gradlew :payments-ui-core:lintRelease :payments-ui-core:detekt :payments-ui-core:apiCheck :paymentsheet:detekt :paymentsheet:apiCheck

Screenshots

Not applicable. The rendered form behavior is unchanged.

Changelog

Not applicable. This is an internal behavior-neutral refactor.