[TheGreatAxios]

Stacked on #68, which is stacked on #67.

Summary

• Replaces the behavior checks test: drop class-string tests covered by browser specs #62 deletes with Playwright specs:
  • Select: value, aria-invalid/aria-describedby, change events and disabled.
  • FilterChip: aria-pressed toggles.
  • DictationStatusLine: denied state shows the default alert copy.
  • AuthLayout: the panel is empty when omitted and renders the node it is given.
• ChatThread: an agent turn renders its reasoning <details> and its tool narrative, and each expands on click.

Verification

bun run check, bun run build and bun run test:e2e pass (15 Playwright tests).

Closes CL-9139