chore: remove undici in favor of global fetch

[volukren]

undici is only used in runtime-tests/node/index.test.ts to call fetch, which has been global since Node 18. The test stack already requires Node >= 20, so the dependency can be dropped and the test can just use the global fetch.

It still appears in bun.lock as a transitive dep, so the version line changes there instead of going away

The author should do the following, if applicable

• Add tests
• Run tests
• bun run format:fix && bun run lint:fix to format the code
• Add TSDoc/JSDoc to document the code

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 79.09%. Comparing base (224d2f5) to head (0b26d63).

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #5168   +/-   ##
=======================================
  Coverage   79.09%   79.09%           
=======================================
  Files         154      154           
  Lines       10783    10783           
  Branches     2253     2253           
=======================================
  Hits         8529     8529           
  Misses       2254     2254           

☔ View full report in Codecov by Harness.
📢 Have feedback on the report? Share it here.

🚀 New features to boost your workflow:

• ❄️ Test Analytics: Detect flaky tests, report on failures, and find test suite problems.
• 📦 JS Bundle Analysis: Save yourself from yourself by tracking and limiting bundle sizes in JS merges.