feat: add OpenAI /v1/completions adapter for vLLM gpt-oss-120b accuracy

[arekay-nv]

Adds APIType.OPENAI_COMPLETIONS routing to /v1/completions, which accepts pre-tokenized token ID arrays and bypasses vLLM's chat template — required for gpt-oss-120b where the Harmony format must be applied client-side.

• Add APIType.OPENAI_COMPLETIONS with default_route "/v1/completions"
• Add TextCompletionRequest/Response/SSE msgspec types
• Add OpenAITextCompletionsAdapter (mirrors SGLang adapter, reuses OpenAISSEAccumulator)
• Register adapter and accumulator in endpoint_client/config.py
• Rename gptoss → gptoss_sglang presets; add gptoss_vllm across aime25/gpqa/livecodebench
• Update sglang_gptoss_120b_example.yaml to use gptoss_sglang presets
• Update vllm_gptoss_120b_example.yaml to use openai_completions + gptoss_vllm presets
• Add 18 unit tests covering adapter, SSE, preset existence, and APIType integration

fix: move lazy test imports to module level; fix decode_sse_message return type

• Move all inline imports in test_completions_adapter.py to file-level
• Add test for empty-text SSE choice path
• Fix HttpRequestAdapter.decode_sse_message abstract annotation from str -> Any (SGLang and completions adapters both return SSEDelta structs, not str)

examples/04_GPTOSS120B_Example/Readme.md:

• Replace stale chat-completions note with accurate openai_completions description
• Update performance-only vLLM api_type reference from "openai" to "openai_completions"

What does this PR do?

Type of change

• Bug fix
• New feature
• Documentation update
• Refactor/cleanup

Related issues

Testing

• Tests added/updated
• All tests pass locally
• Manual testing completed

Checklist

• Code follows project style
• Pre-commit hooks pass
• Documentation updated (if needed)

[github-actions]

MLCommons CLA bot All contributors have signed the MLCommons CLA ✍️ ✅

[gemini-code-assist]

Code Review

This pull request introduces a new openai_completions API type and adapter to support the OpenAI /v1/completions endpoint, enabling the use of pre-tokenized input with vLLM. This change allows users to bypass server-side chat templates, ensuring parity with SGLang results for specific models like gpt-oss-120b. The implementation includes the OpenAITextCompletionsAdapter, updated configuration templates, documentation, and new unit tests. I have no feedback to provide.