use document role for chat responses on windows/linux

[meganrogge]

fix #228599

For mac, using document vs listitem would limit navigation flexibility, so we will stick with listitem.

Using document for windows/linux offers several advantages:

• switches to browse/virtual cursor mode automatically
• indicates to screen readers that the element is a primary content area, enabling document-based navigation (e.g., by headings, landmarks)
• typically enhances the reading experience, especially for text-heavy content
• reduces noise of "listitem" being announced

Some downsides:

• removing listitem might reduce understanding of its context in the conversation hierarchy.
• role=document may require users to switch out of reading mode to interact with buttons or links, adding friction.
• for cross-platform users, the differing roles might be confusing due to inconsistent behavior

cc @jooyoungseo and @rperez030

[meganrogge]

also cc @Tyriar in case you have any thoughts here

[jooyoungseo]

@meganrogge I think we need to test this PR with NVDA and JAWS to see if they behave as expected. I was wondering if users would still be able to navigate between next and previous responses in the Chat thread. Users may want to use Up/DownArrows to navigate through response items and hit Enter to trigger the "document" view. ESC to go back to the list view. This concept aligns with Outlook's mail preview list (Up/DownArrows) -> displaying full content (with Enter) -> going back to the preview list (via ESC).