Follow up: Confirm SegmentButton default size

## Overview

Follow-up from #1172 to confirm the intended default size for `SegmentButton`.

## Problem

`SegmentButton` currently defaults to `ButtonBaseSize.Sm`, but review feedback on #1172 suggested `ButtonBaseSize.Md` because most segment buttons use the medium size.

This should be resolved explicitly so the implementation, docs, stories, and design guidance all agree on the default size.

## Acceptance Criteria

- [ ] Confirm whether the default `SegmentButton` size should be `Sm` or `Md`.
- [ ] If the default should be `Md`, update the component default and add/update tests.
- [ ] If the default should remain `Sm`, document that decision in the README or related design guidance.
- [ ] Ensure Storybook default args reflect the intended default behavior.

## References

- PR: https://github.com/MetaMask/metamask-design-system/pull/1172
- Review comment from Amanda: suggested `size = ButtonBaseSize.Md`
- File: `packages/design-system-react-native/src/components/SegmentButton/SegmentButton.tsx`

Start with packages/design-system-react-native/src/components/SegmentButton/SegmentButton.tsx and review PR #1172 and its referenced feedback. Determine whether the default should be Sm or Md, then align the component, tests, Storybook default args, and README or design guidance with that decision.

Written by the indexing model from the issue text.