fix#139486
The `expandedInsets` property enhancement for the `SegmentedButton` widget introduces flexibility in button layout design within Flutter applications. When `expandedInsets` is not null, this property allows the `SegmentedButton` to expand, filling the available horizontal space of its parent container and also have the specified insets, thus ensuring a uniform distribution of segment widths across the button. Conversely, with `expandedInsets` is null, the widget sizes itself based on the intrinsic sizes of its segments, preserving the natural width of each segment. This addition enhances the adaptability of the `SegmentedButton` to various UI designs, enabling developers to achieve both expansive and compact button layouts seamlessly.
### Setting expandedInsets = null
<img width="724" alt="image" src="https://github.com/flutter/flutter/assets/65075121/f173b327-a34b-49f0-a7b1-a212a376c5e3">
### Setting expandedInsets = EdgeInsets.zero
<img width="724" alt="image" src="https://github.com/flutter/flutter/assets/36861262/f141a3d3-80e3-4aeb-b7c8-d56ca77ca049">
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