[Swift] Respect Retry-After for preload throttling

[markmur]

What changes are you making?

• Parse Retry-After delay-seconds and HTTP-date response headers.
• Add a managed preload throttle policy by default: a 429 with a valid Retry-After value returns a throttled failure and suppresses explicit preload calls until the delay elapses.
• Add a passthrough policy that returns the existing HTTP failure with retryAfter metadata and applies no cooldown.
• Expose retryAfter on presentation CheckoutError values without changing presentation behavior.
• Document the policies and update the public Swift API baseline.

Checkout Kit never retries a throttled preload automatically, and preload cooldowns never block presentation.

Before you merge

• I have added tests to support my implementation.
• I have updated the Swift README.

RetryAfterTests, ConfigurationTests, PreloadObservabilityTests, CheckoutWebViewTests, the Swift public API check, SwiftLint, and SwiftFormat pass. CocoaPods lint could not run because the local Ruby bundle is missing installed gems.

[markmur]

• [Android] Respect Retry-After for preload throttling #661
• [Swift] Respect Retry-After for preload throttling #660 👈 (View in Graphite)
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