[wip] feature(specs): add OpenResponses Spec for Responses API (1/n)

[bhimrazy]

What does this PR do?

Adds a new OpenResponsesSpec class to implement the OpenResponses API specification for multi-provider, interoperable LLM interfaces. This initial implementation supports basic non-streaming text responses, following the OpenResponses specification.

A part of #455

Key changes:

• New OpenResponsesSpec in src/litserve/specs/open_responses.py
• New types in src/litserve/types.py for roles and statuses
• Unit test in tests/unit/test_open_responses.py for non-streaming responses

Future PRs can extend to full compliance. This provides a solid foundation for incremental development.

PR review

Anyone in the community is free to review the PR once the tests have passed.

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[codecov]

Codecov Report

❌ Patch coverage is 84.68468% with 17 lines in your changes missing coverage. Please review.
✅ Project coverage is 85%. Comparing base (d244f2c) to head (58fbd7f).

Additional details and impacted files

@@         Coverage Diff          @@
##           main   #650    +/-   ##
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  Coverage    85%    85%            
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  Files        39     41     +2     
  Lines      3212   3323   +111     
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+ Hits       2721   2819    +98     
- Misses      491    504    +13     

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