[Frontend][last/N] Move all non-OpenAI content out of the OpenAI folder.

[noooop]

Purpose

Following #41907 #52131 #53500 #53659 #54492

In the early days of vLLM (2023), there was only the OpenAI, so vLLM online serving was referred to as the OpenAI-Compatible Server. As we have entered 2026, generative AI has grown significantly, and more and more features have been added to vLLM online serving.

We'd better move all non-OpenAI content out of the OpenAI folder.

• mv vllm.entrypoints.openai.sse_keep_alive vllm.entrypoints.serve.utils.sse_keep_alive
• rm tests/entrypoints/openai/test_async_tokenization.py [Frontend] Remove AsyncMicrobatchTokenizer. #45759

Test Plan

Test Result

---

Essential Elements of an Effective PR Description Checklist

• The purpose of the PR, such as "Fix some issue (link existing issues this PR will resolve)".
• The test plan, such as providing test command.
• The test results, such as pasting the results comparison before and after, or e2e results
• (Optional) The necessary documentation update, such as updating supported_models.md and examples for a new model.

[claude]

Claude Code Review

This pull request is from a fork — automated review is disabled. A repository maintainer can comment @claude review to run a one-time review.

[DarkLight1337]

/ci run

[github-actions]

✅ Triggered Buildkite CI #88246 for commit 112eef51be26.