[form] Fix validation bugs

[atomiks]

This fixes a small batch of Form bugs found during review.

Changes

• Allow submit to continue when async field validation is still pending unless a field is already explicitly invalid.
• Reset Base UI field validation and state after native form reset.
• Make Form docs snippets safer by JSON-stringifying fetch bodies and null-checking actionsRef examples.

Original findings addressed

• [P2] Async validators could block the first valid submit.
• [P2] Native reset did not clear Base UI field state.
• [P3] Public docs examples had copy-paste issues.

[pkg-pr-new]

• base-ui-tanstack-start

• vite-css-base-ui-example

  pnpm add https://pkg.pr.new/mui/base-ui/@base-ui/react@4894
  

  pnpm add https://pkg.pr.new/mui/base-ui/@base-ui/utils@4894
  

commit: 49870ba

[code-infra-dashboard]

Bundle size

Bundle	Parsed size	Gzip size
@base-ui/react	🔺+262B(+0.06%)	🔺+100B(+0.07%)

Details of bundle changes

Performance

Total duration: 1,167.09 ms -81.93 ms^(-6.6%) | Renders: 50 ⁽⁺⁰⁾ | Paint: 1,806.36 ms -89.03 ms^(-4.7%)

Test	Duration	Renders
Slider mount (300 instances)	156.97 ms ▼-44.81 ms(-22.2%)	3 (+0)

11 tests within noise — details

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

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