fix: use text/javascript MIME type for local-config.js endpoint

[AntoineDeveloppeur]

** Description of problem

After following the self-hosting setup steps, one of the E2E tests fails.
The issue is caused by this command:

cp public/test/local-config.example.js public/test/local-config.js

Screenshot of failed test:

The failure happens because MIME type of js file is 'text/javascript' the test needs 'application/javascript'
When test is run again if fails because the copied file contains text, but the test expects the file to be empty (line 65)

** Fix

• If there is no file, worker now responds with an empty file with 'text/javascript'
• The test now expects 'text/javascript' instead of 'application/javascript'
• The test no longer require the file to be empty anymore, allowing it to pass whether the file exist or not. The original intention of emiting missing-resource console errors is preservered.

All tests pass
No documentation update is needed

[neonwatty]

Reviewed with three independent sub-agents. Added deterministic route coverage for the local-config fallback/pass-through behavior; CI and local E2E are green.

[github-actions]

🎉 This PR is included in version 1.37.2 🎉

The release is available on GitHub release

Your semantic-release bot 📦🚀

[neonwatty]

Thanks for fixing this @AntoineDeveloppeur !