fix: try to fix two flaky tests

[MathyV]

Summary

I noticed while developing the other PR. These tests don't fail every time, heavy CPU usage causes them to fail more often since it is more a timing issue then anything else.

Changes

• The keep-alive test used the same Response object for all fetch calls, so depending on the timing, a second call either failed or not. Now each call gets its own Response object.
• The config route test wrote the session-secret file into the working directory but didn't wait for the unlink to complete, so the file could outlive the test. It is put into the temp directory now and properly removed.

Related issues

None

Type of change

• Bug fix (non-breaking change that fixes an issue)
• New feature (non-breaking change that adds functionality)
• Breaking change (fix or feature that would cause existing functionality to change)
• Documentation update
• Refactor / code quality improvement
• Chore / dependency update / CI change

Checklist

• I have read the Contributing Guide
• My code follows the project's code style and conventions
• I have run npm run typecheck && npm run lint and there are no errors
• The build passes (npm run build)
• I have tested my changes locally
• I have added or updated documentation if needed
• I have updated translations (locales/) if my changes affect user-facing text
• I have included screenshots or a screen recording for UI changes

Screenshots / demo

None

Notes for reviewers

None