add litserve.api tests

[aniketmaurya]

Before submitting

• Was this discussed/agreed via a Github issue? (no need for typos and docs improvements)
• Did you read the contributor guideline, Pull Request section?
• Did you make sure to update the docs?
• Did you write any new necessary tests?

⚠️ How does this PR impact the user? ⚠️

Describe (in plain English, not technical Jargon) how this improves the user experience. If you can't tie it back to a real tangible, user goal or describe it in plain english, it's a hint that this is likely not needed and is probably an "engineering nit".

GOOD:

"As a user, I need to serve models faster. This PR focuses on enabling speed gains by using GPUs"

BAD:

"This PR enables GPUs".
This is bad because the user problem is not clear... instead it just jumps to the solution without any context.

PRs without this will not be merged.

---

What does this PR do?

litserve.api has 78% test coverage, this PR adds test for untested codes.

Before

After

PR review

Anyone in the community is free to review the PR once the tests have passed.
If we didn't discuss your PR in GitHub issues there's a high chance it will not be merged.

Did you have fun?

Make sure you had fun coding 🙃

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 87%. Comparing base (07d60df) to head (511832b).
Report is 1 commits behind head on main.

Additional details and impacted files

@@         Coverage Diff         @@
##           main   #252   +/-   ##
===================================
+ Coverage    85%    87%   +1%     
===================================
  Files        14     14           
  Lines      1078   1078           
===================================
+ Hits        921    933   +12     
+ Misses      157    145   -12