CHECKOUT-9513: Add option to pass initial state during initialisation

[davidchin]

What/Why?

This PR introduces the ability to pre-populate the Checkout SDK with initial data during service creation, eliminating the need for client-side API calls on page load. Currently, the checkout microapp fetches all data client-side after the page loads, which is suboptimal because these requests can only be made after assets are loaded and parsed, and the page often already has this same data available.

// Before
const service = createCheckoutService();
await service.loadCheckout();

// After
const service = createCheckoutService();
await service.hydrateInitiateState({ checkout, config, formFields, extensions });

Rollout/Rollback

Revert or turn off CHECKOUT-9388.initial_state_for_checkout_app experiment

Testing

Before

This video shows that, before this change, the page required AJAX requests on initial load.

Screen.Recording.2025-09-29.at.1.20.44.pm.mov

This screenshot shows that the LCP is 2.3s before this change.

After

This video shows that, after this change, the page no longer makes AJAX requests during the initial load.

Screen.Recording.2025-09-29.at.1.09.04.pm.mov

This screenshot shows that the LCP is 1.2s after this change.