Add a semantics support for to the line chart tooltip in LineTooltipItem

The current implementation of the line chart includes semantics for the entire chart but lacks semantics for the tooltip popup that appears when hovering over data points. This limitation prevents screen readers from picking up the information displayed in the tooltip, affecting the chart's accessibility for visually impaired users.

Current Behavior: The screen reader reads the entire chart but does not pick up any information in the tooltip popup.

Expected Behavior: The screen reader should read the tooltip information when a user hovers over or selects a data point in the line chart.

Start by locating the LineTooltipItem implementation and reading how the line chart currently exposes semantics for the overall chart and tooltip popup. Verify the change by checking that screen readers can receive tooltip information when a data point is hovered over or selected.

Written by the indexing model from the issue text.