This PR makes sure we do not select beyond the text boundary at the tapped position unless, we tap at the end of the text which in that case we should select the previous text boundary.

```dart
    // if x is a boundary defined by `textBoundary`, most textBoundaries (except
    // LineBreaker) guarantees `x == textBoundary.getLeadingTextBoundaryAt(x)`.
    // Use x - 1 here to make sure we don't get stuck at the fixed point x.
    final int start = textBoundary.getLeadingTextBoundaryAt(extent.offset - 1) ?? 0;
```

This was originally carried over from https://github.com/flutter/flutter/blob/f468f3366c26a5092eb964a230ce7892fda8f2f8/packages/flutter/lib/src/widgets/editable_text.dart#L4167-L4179 which used this `x - 1` to be able to move to the previous word boundary when navigating with a keyboard. When selecting by tapping/clicking we do not want to move past the text boundary at the tapped position so this adjustment is not needed.

Fixes #132126