test(forms): cover states and interactions

[Fabluchy]

Summary

Add comprehensive React Testing Library tests for the TextField and Button form components covering loading, empty, error, success, disabled, busy, and keyboard interaction states. Asserts accessible names and roles.

Closes #561

Changes

TextField (previously 1 test → now 35 tests)

• Error states: role=alert, aria-invalid, aria-describedby, error clearing/replacement
• Loading/busy states: aria-busy forwarding, disabled+aria-busy combined
• Empty state: no value displayed, no aria-invalid when clean
• Disabled state: disabled attribute, disabled blocks value changes
• Keyboard interaction: value changes via fireEvent.change, controlled/uncontrolled values
• Accessibility: label htmlFor association, focus-visible outline, no hardcoded colors
• HTML attribute forwarding: name, type, required, maxLength, inputMode, className
• Success state: aria-invalid removed when error cleared

Button (previously 8 tests → now 29 tests)

• Disabled state: disabled attr, disabled styling, click prevention, all variants
• Loading/busy states: aria-busy forwarding, disabled+aria-busy combined, accessible name preserved
• Click handling: single click, multi-click, disabled click prevention
• Variants: primary, secondary, danger with correct class assertions
• Type attribute: button, reset forwarding
• HTML attribute forwarding: name, value, className merging
• Existing asChild and focus ring tests preserved

Test Output

```
Test Suites: 2 passed, 2 total
Tests: 64 passed, 64 total
Snapshots: 0 total
Time: ~1.4s
```

Checklist

• `npm run lint` passes
• `npm test` — 64/64 tests pass
• `npm run build` passes
• No behavior changes — tests only