Fix accelerated checkout button hit area

[markmur]

Summary

• remove the default flex: 1 style from AcceleratedCheckoutButtons so the native onSizeChange height participates in Yoga layout
• leave the native component unstyled until it reports a height, then apply only that height
• update the regression test to ensure flex sizing cannot override the reported height again

Reproduction

Reproduced with Checkout Sheet Kit 3.8.1, Expo SDK 57, React Native 0.86.0, and an iOS 18 simulator. With two wallets, native reported 104 pt while the React Native frame remained 48 pt; taps on the visibly overflowing button edges were dropped. Removing flex: 1 made the React Native frame match the reported height.

Fixes #502

[github-actions]

Coverage Report

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