Fix polymiddleware error should not propagate to React runtime

[compulim]

Changelog Entry

Fixed

• Fixed polymiddleware error should not propagate to React runtime if <DebugProvider> is not mounted, by @compulim, in PR #5833

Description

<ErrorBoxPolymiddlewareProxy> and useBuildRenderErrorBoxCallback should have "catchall". Without<DebugProvider>, rendering those component should not throw.

Design

Specific Changes

• Added a catchall which will null-render if <DebugProvider> or any error box polymiddleware is not added

• 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)

[Copilot]

Pull request overview

This PR prevents error box polymiddleware “fallthrough” warnings/errors from propagating into the React runtime when <DebugProvider> is not mounted, by adding a null-rendering catchall fallback and an HTML2 regression test.

Changes:

• Added a NullComponent catchall fallback to ErrorBoxPolymiddlewareProxy and to useBuildRenderErrorBoxCallback()-built render callbacks.
• Added an HTML2 test to validate Adaptive Cards render failures don’t produce the RCoR fallthrough warning when <DebugProvider> is absent.
• Added a changelog entry documenting the fix.

Reviewed changes

Copilot reviewed 3 out of 4 changed files in this pull request and generated 1 comment.

File	Description
packages/api-middleware/src/errorBoxPolymiddleware.tsx	Adds a null fallback catchall for error box proxy and the render-callback builder hook to avoid RCoR fallthrough surfacing to React runtime.
tests/html2/adaptiveCard/renderError.withoutDebug.html	Adds regression coverage for rendering a failing Adaptive Card without <DebugProvider>.
CHANGELOG.md	Documents the fix in the changelog.

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