Hide the default avatar initials from screen readers in production builds

[Eugene (OEvgeny)]

Related to #5803

Changelog Entry

Fixed

• Fixed screen readers reading the avatar initials, such as "JO", before every message in production builds since 4.19.0: the default avatar is hidden from assistive technologies again, related to #5803, in PR #5899, by @OEvgeny

Description

Since #5779, DefaultAvatar relies on the valibot schema to default aria-hidden to true. validateProps() does not parse props in production builds, so the default is never applied and screen readers read the initials before every message (the "BO Bot JO said:" in #5803). The default is now set in the component again, as before 4.19.0.

Design

• Defaults stay in the component (destructuring after validateProps()), like other components, and the schema no longer declares a default that production builds never apply.
• Open question: should validateProps(…, 'strict') apply schema defaults in production too? AbsoluteTime, RovingTabIndexComposer, and Fluent's ActivityLoader also declare schema defaults; they are never applied in any build, but their callers do not rely on them.

Specific Changes

• DefaultAvatar.tsx: aria-hidden defaults to true in the component instead of the schema.

• DefaultAvatar.spec.tsx: new unit test that renders DefaultAvatar with validateProps() as in production builds; html2 tests run development builds.

• I have added tests and executed them locally

• I have updated CHANGELOG.md

• I have updated documentation

Review Checklist

This section is for contributors to review your work.

• Accessibility reviewed (tab order, content readability, alt text, color contrast)
• Browser and platform compatibilities reviewed
• CSS styles reviewed (minimal rules, no z-index)
• Documents reviewed (docs, samples, live demo)
• Internationalization reviewed (strings, unit formatting)
• package.json and package-lock.json reviewed
• Security reviewed (no data URIs, check for nonce leak)
• Tests reviewed (coverage, legitimacy)

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