fix: improve request type handling for unannotated endpoints in Swagger

[jwliu36]

Fixes #667

What does this PR do?

As a user, I want to use the Swagger UI (/docs) to test my LitServe endpoints interactively. Before this fix, the /predict endpoint showed no input form in Swagger when decode_request had no type annotation, so clicking "Execute" sent an empty body and crashed the server with a JSONDecodeError. Now Swagger correctly renders a JSON request body form for these endpoints.

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?

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[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 85%. Comparing base (8fb245c) to head (912bfb4).
⚠️ Report is 5 commits behind head on main.

Additional details and impacted files

@@         Coverage Diff         @@
##           main   #676   +/-   ##
===================================
  Coverage    85%    85%           
===================================
  Files        39     39           
  Lines      3278   3283    +5     
===================================
+ Hits       2774   2779    +5     
  Misses      504    504           

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