fix(vite-plugin): normalize wrangler asset directory path separators for Windows compatibility

[shulaoda]

Fixes #13185.

• Normalize the return value of getAssetsDirectory() with vite.normalizePath() to ensure assets.directory in the output wrangler.json always uses forward slashes
• On Windows, path.relative() returns backslash-separated paths (e.g. ..\client), producing invalid paths in the platform-agnostic config file

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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: windows compatibility

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Latest commit: 3bee0f9

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• ✅ @cloudflare/wrangler

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commit: 3bee0f9

[jamesopstad]

Thanks for the PR!