refactor: Addressed follow up for ListItem

[brianacnguyen]

Description

Addresses review follow-ups from #1231 for the React Native ListItem / Content accessory refactor.

Why: The original refactor moved row shell accessories to ListItem and hardcoded gap={0} on the internal BoxRow, removing the previous 16px spacing. Review also called for a MIGRATION.md entry for the breaking prop removals and symmetric test coverage for endAccessory.

What changed:

• accessoryGap prop — New ListItem prop (BoxSpacing, default 0) controls spacing between startAccessory / endAccessory and the inner Content row. Pass accessoryGap={4} to restore legacy 16px spacing.
• MIGRATION.md — Added 0.29.0 → 0.30.0 entry documenting:
  • startAccessory / endAccessory moved from Content to ListItem
  • topAccessory / bottomAccessory removed; compose with BoxColumn instead
  • accessoryGap={4} note for restoring previous spacing
• Tests — Added standalone endAccessory + avatar test (symmetric with existing startAccessory coverage)
• Docs/stories — Documented accessoryGap in README; added AccessoryGap story and Storybook control; updated StartAccessory / EndAccessory stories to use accessoryGap={4}

Related issues

Follow-up to #1231

Fixes:

Manual testing steps

1. Run React Native Storybook: yarn storybook:ios or yarn storybook:android
2. ListItem
   • Verify StartAccessory, EndAccessory, and AccessoryGap stories
   • Compare AccessoryGap story rows — accessoryGap={0} vs accessoryGap={4} spacing should be visibly different
   • Toggle accessoryGap in Storybook controls on accessory stories
3. Run tests:

   yarn workspace @metamask/design-system-react-native run jest src/components/ListItem/ListItem.test.tsx

Screenshots/Recordings

Before

ListItem hardcoded gap={0} between shell accessories and inner content with no way for consumers to adjust spacing.

After

Simulator.Screen.Recording.-.iPhone.15.Pro.Max.-.2026-06-12.at.15.03.50.mov

Pre-merge author checklist

• I've followed MetaMask Contributor Docs
• I've completed the PR template to the best of my ability
• I've included tests if applicable
• I've documented my code using JSDoc format if applicable
• I've applied the right labels on the PR (see labeling guidelines). Not required for external contributors.

Pre-merge reviewer checklist

• I've manually tested the PR (e.g. pull and build branch, run the app, test code being changed).
• I confirm that this PR addresses all acceptance criteria described in the ticket it closes and includes the necessary testing evidence such as recordings and or screenshots.

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