[checkbox] Remove redundant validation input ref

[nami8824]

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

My understanding is that validation.inputRef is redundant here.

Checkbox.Root already registers its hidden input through validation.registerInput, and validation uses the registered inputs first before falling back to validation.inputRef.

[pkg-pr-new]

• base-ui-tanstack-start

• vite-css-base-ui-example

  pnpm add https://pkg.pr.new/mui/base-ui/@base-ui/react@5188
  

  pnpm add https://pkg.pr.new/mui/base-ui/@base-ui/utils@5188
  

commit: 04b53e7

[code-infra-dashboard]

Bundle size

Bundle	Parsed size	Gzip size
@base-ui/react	▼-12B(0.00%)	▼-6B(0.00%)

Details of bundle changes

Performance

Total duration: 1,327.72 ms +43.78 ms^(+3.4%) | Renders: 78 ⁽⁺⁰⁾

Test	Duration	Renders
Slider mount (300 instances)	215.81 ms 🔺+57.29 ms(+36.1%)	3 (+0)

13 tests within noise — details

Metric alarms

Test	Metric	Change
Slider mount (300 instances)	bench:paint	🔺 +85.72 ms

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[atomiks]

Thanks @nami8824