Fix flaky Swift preload expiry test

[tiagocandido]

Summary

• give the preload expiry test a 1s freshness margin instead of 0.1s
• extend the expiry polling timeout to 3s so slow CI has room for the expiry timer
• keep production preload cache behavior unchanged

Why

The test created an entry with only 0.1s of TTL remaining, then immediately asserted the cache still existed. On slow CI, the entry could become stale before those assertions and invalidate itself.

Test plan

• shadowenv exec --dir /Users/tiagosantos/src/github.com/Shopify/checkout-kit -- /opt/dev/bin/dev swift test CheckoutWebViewTests
• shadowenv exec --dir /Users/tiagosantos/src/github.com/Shopify/checkout-kit -- /opt/dev/bin/dev swift test CheckoutWebViewTests

[github-actions]

React Native — Coverage Report

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	91.66% (319/348)	87.86% (181/206)	100% (82/82)