Why are these changes needed?

This PR modifies the setup-dev.py script to preserve ray/serve/generated folder during the symlink creation process. Specifically:

• Before creating the symbolic link for serve, the existing generated folder is copied to a temporary location.
• After the symlink is created, the generated folder is moved back to its original location, ensuring that the generated files are preserved and not overwritten by the symlink.

Related issue number

Closes #48792

Checks

• I've signed off every commit(by using the -s flag, i.e., git commit -s) in this PR.
• I've run scripts/format.sh to lint the changes in this PR.
• I've included any doc changes needed for https://docs.ray.io/en/master/.
  • I've added any new APIs to the API Reference. For example, if I added a
    method in Tune, I've added it in doc/source/tune/api/ under the
    corresponding .rst file.
• I've made sure the tests are passing. Note that there might be a few flaky tests, see the recent failures at https://flakey-tests.ray.io/
• Testing Strategy
  • Unit tests
  • Release tests
  • This PR is not tested :(