Follow up: Align SectionHeader titleWrapperProps gap contract

Overview

Follow-up from #1175 (feat: [DSRN] Added SectionHeader).

SectionHeaderProps['titleWrapperProps'] is typed and documented as a passthrough to the inner BoxRow, excluding only children, endAccessory, and textProps.

Because gap is not omitted from the type, consumers can pass:

<SectionHeader
  title="Assets"
  titleWrapperProps={{ gap: 2 }}
/>

But SectionHeader.tsx spreads titleWrapperProps before gap={1}, so the explicit prop is overwritten and has no runtime effect.

Acceptance Criteria

• Decide whether inner title-row gap should be configurable.
• If configurable, spread titleWrapperProps after default row props while keeping controlled fields (children, endAccessory, textProps) protected.
• If not configurable, omit gap from titleWrapperProps and update the README type table/description to match.
• Add or update a focused test that locks in the chosen contract.

References

• PR: https://github.com/MetaMask/metamask-design-system/pull/1175
• Current implementation: packages/design-system-react-native/src/components/SectionHeader/SectionHeader.tsx
• Docs: packages/design-system-react-native/src/components/SectionHeader/README.md

Start with packages/design-system-react-native/src/components/SectionHeader/SectionHeader.tsx and compare its titleWrapperProps typing and spread order with the README type table in packages/design-system-react-native/src/components/SectionHeader/README.md. Confirm the intended gap contract, then update the implementation or documentation and add or update a focused SectionHeader test that verifies the chosen behavior.