Data table tooltip truncation full contents not available using keyboard

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### Summary of the bug
Data table cells containing data which is truncated using the tooltip pattern can't be expanded and read in full using just a keyboard.

### 🧐 Expected behaviour
Users should be able to expand the full contents of a truncated cell using only a keyboard. It should be possible to achieve this by making the cell focusable and displaying the tooltip on focus.

### 📝 Acceptance Criteria
The contents of a data table cell which has been truncated using the tooltip pattern can be expanded and read in full using only a keyboard.

### 🚨 Urgency (low, medium or high)
If applicable, tell us how urgent it is that this issue gets resolved, based on the impact it has on your team's work or project timeline.

Start by locating the data table cell truncation and tooltip implementation in the TypeScript component source, then inspect any related tests. Verify the current keyboard behavior and update the component so truncated cell contents can be focused and expanded for keyboard users; the acceptance criterion is that the full contents can be read using only a keyboard.

Written by the indexing model from the issue text.