Fixes #154253

Before this change the _rect used by _SelectableFragment was calculated using getBoxesForSelection which defaults to a BoxHeightStyle of BoxHeightStyle.tight. This style ensures the boxes returned from getBoxesForSelection tightly wrap each character. This was causing an issue when text ended or was split on new lines because the SelectableFragment._rect did not include the region that actually had the new lines. So when _SelectableFragment internally used SelectionUtils.adjustDragOffset to clamp the position to the selectable rect, the selectable rect is inaccurate and always results in a position that is above the new line and not on it.

After this change the _rect used by _SelectableFragment is calculated using getBoxesForSelection with a BoxHeightStyle of BoxHeightStyle.max. This style ensures the boxes returned from getBoxesForSelection cover the entire height of the line, and as a result including the new line character region.

Other notes:

TextPainter.getBoxesForSelection explicitly notes that leading or trailing new line characters will be represented by zero-width boxes.

Pre-launch Checklist

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• I listed at least one issue that this PR fixes in the description above.
• I updated/added relevant documentation (doc comments with ///).
• I added new tests to check the change I am making, or this PR is [test-exempt].
• I followed the [breaking change policy] and added [Data Driven Fixes] where supported.
• All existing and new tests are passing.