Assert batch size to strictly test OpenAIEmbeddingSpec for dynamic batching

[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?

Improved test = reliability

This PR adds an assert to the batched input in the LitAPI to strictly confirm it's always batched when enabled.

PR review

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[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 94%. Comparing base (771a1c9) to head (a9a60bf).
Report is 2 commits behind head on main.

Additional details and impacted files

@@         Coverage Diff         @@
##           main   #370   +/-   ##
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  Coverage    94%    94%           
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  Files        25     25           
  Lines      1563   1563           
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  Hits       1465   1465           
  Misses       98     98