test(opentui): expand cli and input coverage

[benvinegar]

Summary

• add CLI tests for help, formatting flag precedence, and argument validation
• add direct input-handler coverage for keyboard shortcuts, text entry, and palette mouse interactions
• add targeted layout/input tests that raise coverage across the app shell without changing runtime behavior

Testing

• bun test packages/app/src/main.test.ts packages/opentui/src/app/input.test.ts packages/opentui/src/app/layout.test.ts
• bun test --coverage

Coverage

• overall coverage improved from 82.43% funcs / 80.02% lines to 87.30% funcs / 85.74% lines
• packages/opentui/src/app/input.ts improved from 25.00% funcs / 10.59% lines to 85.71% funcs / 63.28% lines

This PR description was generated by Pi using openai/gpt-5