Improve accessibility labels and CSS styling for agent session sections (#305604)

* Improve accessibility labels for agent session sections and enhance CSS styling for section labels

* Enhance accessibility tests for agent session sections by updating aria label checks for singular and plural session counts

Co-authored-by: Copilot <[email protected]>

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Co-authored-by: mrleemurray <[email protected]>
Co-authored-by: Copilot <[email protected]>