WIP: Add style option to configure default group open state

[OEvgeny]

Fixes #

Changelog Entry

Description

Design

Specific Changes

• 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)

[OEvgeny]

@copilot update the test files related to the following files adding "When (copilot): opening the group" action after the first of activities in the group lands and before the snapshot is taken. The different groups can be distinguished by isPartOf.@id. If these ids don't match, this means the messages belong to different groups.

__tests__/html2/attachment/adaptiveCard/custom.copilot.html
__tests__/html2/part-grouping/copilot.dark.html
__tests__/html2/part-grouping/copilot.html
__tests__/html2/part-grouping/folded.copilot.html
__tests__/html2/part-grouping/folded.copilot.dark.html

The code to toggle the first group on the page is:

      await host.click(document.querySelector('.collapsible-grouping__toggle'));

[Copilot]

@OEvgeny I've opened a new pull request, #5772, to work on those changes. Once the pull request is ready, I'll request review from you.