fix(checkout): CHECKOUT-10180 Initialize checklist formik with correct value

[bc-maxy]

What/Why?

This PR fixes the issue when a checkout page is rendered without pre-selected shipping method even though it comes as selected through our initial state.

The issue is that Checklist forwards defaultSelectedItemId to Accordion for visual pre-selection, but never syncs that value into its own Formik field.

Fix: the existing useEffect that runs on mount and unmount now also seeds the Formik field with defaultSelectedItemId on mount, if present.

This Checklist component is also used by Payment method selection, but this PR should not make any difference there. In Payment method selection defaultSelectedItemId comes from Formik, so setting it on mount will be a no-op.

Added tests covering both the seeded and empty-default cases.

Rollout/Rollback

Revert the PR.

Testing

New CI tests + manual testing:

Before

Screen.Recording.2026-07-07.at.2.07.26.PM.mov

After

Screen.Recording.2026-07-07.at.2.12.47.PM.mov

---

Note

Low Risk
Small UI form sync change in a shared component; payment paths are expected to be no-ops when the value already comes from Formik.

Overview
Fixes checkout shipping (and similar checklist UIs) where the accordion looked pre-selected but the Formlist/Formik field stayed empty because defaultSelectedItemId was only passed to Accordion, not written into Formik.

On mount, the existing cleanup useEffect now also calls setFieldValue(name, defaultSelectedItemId) when that prop is set, so initial server/checkout state matches the form value. Unmount still clears the field. Payment flows that already derive the default from Formik should behave the same.

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[bc-maxy]

cursor review

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[bc-peng]

Nice detective job👏👏