[menu] Support group labels in radio groups

[nami8824]

Summary

• Allow Menu.GroupLabel to label Menu.RadioGroup.
• Update the group labels docs and demos to show Menu.GroupLabel inside Menu.RadioGroup.
• Update the related minified error message.

closes: #1725

• I have followed (at least) the PR section of the contributing guide.

[pkg-pr-new]

• base-ui-tanstack-start

• vite-css-base-ui-example

  pnpm add https://pkg.pr.new/mui/base-ui/@base-ui/react@4826
  

  pnpm add https://pkg.pr.new/mui/base-ui/@base-ui/utils@4826
  

commit: d573ae4

[code-infra-dashboard]

Bundle size

Bundle	Parsed size	Gzip size
@base-ui/react	🔺+117B(+0.03%)	🔺+25B(+0.02%)

Details of bundle changes

Performance

Total duration: 1,640.54 ms +194.62 ms^(+13.5%) | Renders: 50 ⁽⁺⁰⁾ | Paint: 2,469.00 ms +280.16 ms^(+12.8%)

Test	Duration	Renders
Tabs mount (200 instances)	331.67 ms 🔺+57.53 ms(+21.0%)	4 (+0)
Slider mount (300 instances)	240.09 ms 🔺+48.05 ms(+25.0%)	3 (+0)
Menu mount (300 instances)	190.37 ms 🔺+34.15 ms(+21.9%)	2 (+0)
Menu open (500 items)	107.51 ms 🔺+22.29 ms(+26.2%)	12 (+0)
Dialog mount (300 instances)	79.72 ms 🔺+21.82 ms(+37.7%)	1 (+0)

7 tests within noise — details

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