refactor(other): use react 18 concurrent rendering system

[bc-maxy]

What/Why?

Enabling concurrency in checkout-js: https://react.dev/blog/2022/03/29/react-v18#what-is-concurrent-react
React 19 doubles down on concurrency concepts with Actions and React is heading in a direction of making non-concurrent rendering legacy, so we will need to update our repo eventually.
This PR introduces concurrent rendering.

I had to add flushSync(() => setCheckoutState(newCheckoutState)) which is a workaround. From my understanding it's the Checkout SDK that owns which step is complete (via getCheckoutStepStatuses). At the same time we also have activeStepType in its own useState. flushSync hack just forces synchronous re-render to keep those two in sync. We need to properly fix this by eliminating activeStepType local state if possible..

Breaking Change: invalid containerId now throws instead of silently failing

renderCheckout and renderOrderConfirmation previously called ReactDOM.render(..., null) when the container element wasn't found, which only logged a warning. With the React 18 migration to createRoot, both functions now throw an explicit error:

• Unable to find checkout container: #<id>
• Unable to find order confirmation container: #<id>

Action required: Ensure the container element exists in the DOM before calling either function. Any code that relied on silent failure with an invalid containerId (e.g. during SSR or conditional mounting) must now guard the call explicitly.

Rollout/Rollback

Revert the PR.

Testing

No new functionality, so our tests should catch any discrepancies. However, see breaking change description above - CI tests are added to highlight this behavior.

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Note

Medium Risk
Updates core mount/render paths and checkout state subscription behavior to accommodate React 18 concurrent rendering, which can change timing and surface new runtime errors (e.g., missing container IDs). Test updates reduce flakiness but the rendering and state-sync changes affect critical checkout flows.

Overview
Migrates the app entrypoints to React 18 concurrent rendering by replacing legacy ReactDOM.render with createRoot().render() in renderCheckout and renderOrderConfirmation, and now throws explicit errors when the target containerId is missing.

Updates CheckoutProvider subscriptions to wrap setCheckoutState in flushSync to avoid stale step/status reads under concurrent batching, and adjusts credit-card style probing helpers to use createRoot with deferred unmount cleanup.

Refreshes several integration tests to align with React 18 timing (act usage, waitFor/findBy... instead of process.nextTick) and tweaks jest-setup to ignore a narrower set of third-party console warnings.

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[bc-maxy]

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