ToggleGroup: Fix border rounding overflow on selected item

[danielchalmers]

Description

If you enabled the Rounded property it would be fine, but the inherited border-radius from MudTheme would overflow. This fixes that and simplifies the implementation by hiding the overflow instead of calculating the rounding.

Also reverts a fourth toggle button in the example added in #8377 which made the whole thing too cramped when CheckMarks were enabled.

How Has This Been Tested?

visually

Types of changes

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)

Setting border radius in theme:

Using Rounded proeprty:

I was unable to remove the hairline artifact on the edges but I'll try again in the future. My goal today was making it look presentable.

Checklist

• The PR is submitted to the correct branch (dev).
• My code follows the code style of this project.
• I've added relevant tests.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 90.02%. Comparing base (28bc599) to head (af77482).
Report is 27 commits behind head on dev.

Additional details and impacted files

@@            Coverage Diff             @@
##              dev    #8652      +/-   ##
==========================================
+ Coverage   89.82%   90.02%   +0.19%     
==========================================
  Files         412      418       +6     
  Lines       11878    12025     +147     
  Branches     2364     2372       +8     
==========================================
+ Hits        10670    10825     +155     
+ Misses        681      666      -15     
- Partials      527      534       +7     

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[henon]

Thanks Daniel!