Convert BpkCheckbox component to TypeScript

[Copilot]

Overview

This PR migrates the BpkCheckbox component from Flow to TypeScript, improving type safety and developer experience while maintaining full backward compatibility.

Changes

Component Migration

• Converted BpkCheckbox.js to BpkCheckbox.tsx with proper TypeScript types
• Replaced Flow type annotations with TypeScript equivalents
• Updated Props interface to extend InputHTMLAttributes<HTMLInputElement> for better type inference
• Removed PropTypes in favor of TypeScript's compile-time type checking
• Removed Flow-specific comments and suppressions

Test Files

• Migrated all test files to TypeScript:
  • BpkCheckbox-test.tsx
  • accessibility-test.tsx
  • form-test.tsx (added missing label prop and type assertion)
  • themeAttributes-test.ts
• Updated snapshots to match new .tsx file extensions

Examples & Stories

• Converted examples.js to examples.tsx with proper TypeScript types for StatefulCheckbox
• Updated stories.tsx to use modern Storybook pattern for VisualTestWithZoom

Supporting Files

• Migrated index.js to index.ts
• Converted themeAttributes.js to themeAttributes.ts

Type Safety Improvements

The new TypeScript implementation provides:

• Full type inference for all component props
• Compile-time validation of prop types
• Better IDE autocomplete and documentation
• Explicit typing for event handlers and refs

Testing

✅ All 16 tests passing
✅ 12 snapshots updated
✅ No TypeScript compilation errors
✅ No linting errors
✅ No security vulnerabilities (CodeQL: 0 alerts)

Backward Compatibility

This migration maintains 100% backward compatibility. All existing functionality, props, and behavior remain unchanged.

Original prompt

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This section details on the original issue you should resolve

<issue_title>Convert BpkCheckbox to typescript</issue_title> <issue_description>I want to migrate the BpkCheckbox component and all of its tests and storybook examples to typescript</issue_description>

Comments on the Issue (you are @copilot in this section)

Fixes Skyscanner/backpack#4019

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