Add jest-axe baseline accessibility tests for core components

[KshitijShukla30]

While going through the codebase I noticed packages/react already has Jest and @testing-library/react set up but there are no accessibility checks anywhere. Since EmbeddedChat gets embedded into third-party apps, any a11y issues in core components become the host app's problem too.
jest-axe slots in cleanly on top of the existing setup — no major config changes needed.
What I'm proposing:

Add jest-axe as a dev dependency in packages/react
Extend Jest with toHaveNoViolations
Write baseline tests for ChatInput, MessageList, and Sidebar
Hook into existing CI so it runs on every PR

I already have this working locally. Happy to open a draft PR if this looks good.

[KshitijShukla30]

@Spiral-Memory just wanted to add the reason I think this is worth doing now is that a11y issues are hard to spot in review, a missing label or broken tab order doesn't break anything visually so it just slips through. CI check would catch it automatically. happy to open a draft PR if you think it is correct.