Add React type compatibility tests

[andrewiggins]

Summary

• add compile-time JSX tests for the App Bridge React custom elements
• run the same contract tests with React 18 and pinned React 19 types
• cover representative attributes, element-specific refs, unknown attributes, and invalid refs
• verify the React 19 compatibility fix from Fix React 19 JSX type compatibility #579 through the package's generated public declarations

Testing

• pnpm build
• pnpm type-check
• pnpm test
• pnpm format:check