[serve] add early return when there are still in progress http health check

[GeneDer]

Why are these changes needed?

Currently there is a missing early return when there is a in progress http health check. Due to the default health check period (10s) shorter than the health check timeout (30s), we are always recreating new health check objects and will never fall into the timeout case when the health check hangs. TDD seeing this is the case in the test and seeing adding the missing early return fixes the issue.

Related issue number

Closes #36458

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 :(