[form][field][fieldset] Expand test coverage

[atomiks]

Expands Form, Field, and Fieldset test coverage to 100% across statements, branches, functions, and lines on merged jsdom and Chromium results, and removes the redundant logic the sweep uncovered.

Changes

• Added behavior tests for canceled changes, controlled dirty and touched state, label and description association, error rendering, nested disabled precedence, logical-field validation, and React 17 fallback paths.
• Reused the shared label registration helper for Fieldset.Legend.
• Removed redundant Field.Item parent-checkbox control id plumbing, Field.Error empty fallbacks, and unreachable nullable hidden-input registration paths.

[pkg-pr-new]

• base-ui-tanstack-start

• vite-css-base-ui-example

  pnpm add https://pkg.pr.new/mui/base-ui/@base-ui/react@5281
  

  pnpm add https://pkg.pr.new/mui/base-ui/@base-ui/utils@5281
  

commit: 46aaba5

[code-infra-dashboard]

Bundle size

Bundle	Parsed size	Gzip size
@base-ui/react	▼-117B(-0.03%)	▼-56B(-0.04%)

Details of bundle changes

Performance

Total duration: 1,352.16 ms +66.75 ms^(+5.2%) | Renders: 78 ⁽⁺⁰⁾ | Paint: 2,124.33 ms +87.92 ms^(+4.3%)

No significant changes — details

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Check out the code infra dashboard for more information about this PR.

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