feat: migrate ButtonIcon to ADR-0003 and ADR-0004 (DSYS-485)

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Description

Migrates the ButtonIcon component to follow ADR-0003 (String Unions) and ADR-0004 (Centralized Types Architecture).

Changes:

• Creates ButtonIconSize and ButtonIconVariant const objects in @metamask/design-system-shared (ADR-0003), replacing the duplicate TypeScript enums in both platform packages
• Creates ButtonIconPropsShared type in shared package (ADR-0004) with platform-independent props (iconName: string, size, isDisabled, variant)
• Updates ButtonIconProps in both React and React Native to import and extend from shared (platform-specific props like iconProps: Partial<IconProps>, ariaLabel, className/twClassName, and style remain in platform packages)
• Updates index.ts files in both platform packages to re-export const objects from shared
• Removes old ButtonIconSize and ButtonIconVariant enums from platform types/index.ts files
• Updates all consumers (ButtonIcon.tsx, ButtonIcon.constants.ts, ButtonIcon.stories.tsx, ButtonIcon.test.tsx, BannerBase.tsx) to import from @metamask/design-system-shared

Related issues

Fixes: https://consensyssoftware.atlassian.net/browse/DSYS-485

Manual testing steps

1. Run yarn build — should complete with no errors
2. Run yarn test — all tests pass with 100% coverage
3. Run yarn lint — no errors

Screenshots/Recordings

Before

Duplicate ButtonIconSize and ButtonIconVariant enums defined separately in both design-system-react/src/types/index.ts and design-system-react-native/src/types/index.ts.

After

Single source of truth: ButtonIconSize and ButtonIconVariant as const objects in @metamask/design-system-shared, with ButtonIconPropsShared capturing cross-platform props.

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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