feat: add chat-completions/ and responses/ provider prefixes

[lee-groq]

Summary

Adds chat-completions/ and responses/ prefixes, providing a universal entrypoint for LLM APIs that support Chat Completions or Responses. This allows openbench to support LLM providers that have not been explictly implemented in openbench. Users add a base URL, API key, and prefix their model ID with chat-completions/ or responses/. This is similar to Inspect AI's openai-api/ prefix.

What are you adding?

• Bug fix (non-breaking change which fixes an issue)
• New benchmark/evaluation
• New model provider
• CLI enhancement
• Performance improvement
• Documentation update
• API/SDK feature
• Integration (CI/CD, tools)
• Export/import functionality
• Code refactoring
• Breaking change
• Other

Changes Made

• chat_completions.py, which is a generic format provider for Chat Completions API endpoint
• responses.py, which is a generic format provider for Responses API endpoint
• Relevant updates to the registry, provider config, and docs

Testing

• I have run the existing test suite (pytest)
• I have added tests for my changes
• I have tested with multiple model providers (if applicable)
• I have run pre-commit hooks (pre-commit run --all-files)

Checklist

• My code follows the project's style guidelines
• I have performed a self-review of my own code
• I have commented my code, particularly in hard-to-understand areas
• I have made corresponding changes to the documentation (if applicable)
• My changes generate no new warnings
• I have added tests that prove my fix is effective or that my feature works
• New and existing unit tests pass locally with my changes

Related Issues

Closes #

Additional Context

---

Note

Introduces generic Chat Completions and Responses format providers with new chat-completions/<provider>/<model> and responses/<provider>/<model> prefixes, updates registry/docs/config, and bumps core to 0.5.3.

• Providers:
  • Add ChatCompletionsFormatAPI (src/openbench/model/_providers/chat_completions.py) and ResponsesFormatAPI (src/openbench/model/_providers/responses.py).
  • Register new prefixes in _registry.py as chat-completions and responses.
  • Extend ProviderType and PROVIDER_CONFIGS with generic entries (env vars CHAT_COMPLETIONS_BASE_URL, RESPONSES_BASE_URL, etc.).
• Docs:
  • Update unsupported providers guidance to use chat-completions/<provider>/<model-name> or responses/<provider>/<model-name> and adjust example command.
• Version:
  • Bump openbench-core version to 0.5.3.

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[lee-groq]

Pardon the messy commits, I had to rebase since the version was bumped from 0.5.2 to 0.5.3 after I originally branched