fix(ButtonGroup): apply fullWidth to root element

[YehudaMenahem]

User description

This fix applies the missing fullWidth class to the ButtonGroup root element.
The SCSS already defines .fullWidth { flex: 1 }, which makes sense here since
flex: 1 allows the ButtonGroup to expand proportionally inside its flex container.
This resolves the issue where fullWidth had no effect in Storybook.

• I have read the Contribution Guide.

Resolves #3130

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PR Type

Bug fix

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Description

• Apply missing fullWidth class to ButtonGroup root element

• Enables flex: 1 styling for proportional expansion

• Resolves fullWidth prop having no effect in Storybook

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Diagram Walkthrough

flowchart LR
  A["ButtonGroup component"] -- "add fullWidth class" --> B["Root div with flex: 1 styling"]
  C["fullWidth prop"] -- "conditional rendering" --> B

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File Walkthrough

	Relevant files
Bug fix	ButtonGroup.tsx Add fullWidth class binding to root element                            packages/core/src/components/ButtonGroup/ButtonGroup.tsx Added fullWidth prop to the conditional className binding Applies styles.fullWidth class when fullWidth prop is true Enables the existing SCSS rule flex: 1 to take effect +1/-1

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[qodo-free-for-open-source-projects]

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