[Requires iOS SDK Update] Makes LinkAppearance.PrimaryButtonConfiguration Mapping Consistent With Android

[mliberatore]

Summary

Depends on stripe/stripe-ios#6510 and a subsequent iOS SDK update.

Previously, unspecified values on init made PrimaryButtonConfiguration's properties default to .zero. This led to a mismatch in mapping behavior since the properties couldn’t be nil, and individually apply corner radius or height, falling back to Link defaults for the other.

Motivation

This comment: #2404 (comment)

Testing

• I tested this manually
• I added automated tests

Ensured we were able to set each value independently while relying on a default for the other. I used intentionally obvious values to ensure they were being applied, and differed from sensible defaults.

Corner radius set only	Height set only	Both set	Neither set (both defaults)

Documentation

Select one:

• I have added relevant documentation for my changes.
• This PR does not result in any developer-facing changes.