[unenv-preset] Support native child_process module when experimental flag is enabled

[petebacondarwin]

Support the native node:child_process module from workerd when the enable_nodejs_child_process_module and experimental compatibility flags are enabled.

The native child_process module in workerd (like the unenv polyfill) is a stub - all methods throw errors. The key differences:

• workerd: Throws ERR_METHOD_NOT_IMPLEMENTED (Node.js-style error), validates arguments before throwing
• unenv: Throws generic Error with "[unenv] ... is not implemented yet!", has __unenv__: true marker

Since both implementations are non-functional stubs that throw errors, the breaking change risk is low.

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• Tests
  • Tests included/updated
  • Automated tests not possible - manual testing has been completed as follows:
  • Additional testing not necessary because:
• Public documentation
  • Cloudflare docs PR(s):
  • Documentation not necessary because: This is an experimental feature that follows the existing pattern

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[changeset-bot]

🦋 Changeset detected

Latest commit: 013fe8e

The changes in this PR will be included in the next version bump.

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[claude]

Claude finished @petebacondarwin's task —— View job

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Changeset Review

Reviewing the changeset for this PR...

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✅ All changesets look good

The changeset .changeset/native-child-process-module.md meets all requirements:

• ✅ Version type: minor is correct for adding a new experimental feature
• ✅ Changelog quality: Clear description with explanation of the experimental nature and required compatibility flags
• ✅ Markdown headers: No h1/h2/h3 headers used
• ✅ Experimental feature note: Includes information on how to opt in (requires both enable_nodejs_child_process_module and experimental compatibility flags)

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commit: 013fe8e

[devin-ai-integration]

✅ Devin Review: No Issues Found

Devin Review analyzed this PR and found no potential bugs to report.

View in Devin Review to see 3 additional findings.

[vicb]

Thanks