fix(vite-plugin-cloudflare): avoid The WebSocket is undefined error during build

[edmundhung]

Fixes #8909.

CloudflareDevEnvironment requires initRunner() to be called via the dev plugin's configureServer hook to establish its WebSocket connection.

Frameworks like React Router create a child Vite dev server during builds to compile route files, and strip configureServer from all plugins to avoid conflicts. This means initRunner() never runs, leaving the WebSocket uninitialized. When Vite's dep optimizer or module pruning calls hot.send(), the assertion fails.

The fix checks whether configureServer is still present on the dev plugin at environment creation time. If it's been stripped, we fall back to a standard vite environment.

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• Tests
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  • Automated tests not possible - manual testing has been completed as follows:
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• Public documentation
  • Cloudflare docs PR(s):
  • Documentation not necessary because: bugfix

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commit: daf9a49

[claude]

Claude finished @edmundhung's task —— View job

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✅ All changesets look good

The changeset for .changeset/fix-child-compiler-websocket-assertion.md is properly formatted:

• Version Type: Correctly marked as patch (bug fix)
• Changelog Quality: Clear title and helpful explanation of the issue and scenario
• Markdown Headers: No h1/h2/h3 headers used ✓
• Package: Correctly targets @cloudflare/vite-plugin
• Format: Follows the recommended single-sentence title + explanatory body format

[devin-ai-integration]

✅ Devin Review: No Issues Found

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

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