feat: add nested-when fixture with Playwright tests

[janechu]

Summary

Adds a nested-when fixture to the fast-html test suite demonstrating nested <f-when> directives with SSR hydration.

Changes

packages/fast-html/test/fixtures/when/main.ts

• Added NestedWhenElement class with @observable properties: error, showProgress, enableContinue, clickCount
• Added handleClick arrow function that increments clickCount
• Registered via RenderableFASTElement(NestedWhenElement).defineAsync({ name: 'nested-when', templateOptions: 'defer-and-hydrate' })

packages/fast-html/test/fixtures/when/index.html

• Added <f-template name="nested-when"> with nested <f-when> directives for error/progress/continue-button states
• Added <nested-when id="nested-when"> with pre-rendered shadow DOM containing hydration comment markers
  • Root template UUID: nW0x5kp2Qm

packages/fast-html/test/fixtures/when/when.spec.ts

• Removed .only from an existing test
• Added 5 Playwright tests covering:
  1. Initial state (progress visible, error/button hidden)
  2. Error state (error div + retry button, progress hidden) with full state transitions
  3. showProgress=false shows continue button (not disabled)
  4. Toggling showProgress switches between progress and button
  5. Toggling error switches between error/retry and normal state

Notes

• Tests use await expect(element).not.toHaveAttribute('needs-hydration') before modifying properties to ensure hydration is complete before property changes, avoiding a race condition where pre-rendered DOM is visible before FAST bindings are wired up.