Announce marker contents via aria.alert in goto-error peek widget (#316835)

* Initial plan

* Announce marker via aria.alert when shown in marker peek widget

Co-authored-by: dmitrivMS <9581278+dmitrivMS@users.noreply.github.com>

* Use aria.status instead of aria.alert for marker announcement

Co-authored-by: dmitrivMS <9581278+dmitrivMS@users.noreply.github.com>

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Co-authored-by: copilot-swe-agent[bot] <198982749+Copilot@users.noreply.github.com>
Co-authored-by: dmitrivMS <9581278+dmitrivMS@users.noreply.github.com>