[R3] Add routed experts to openai entrypoint

[hao-aaron]

Purpose

Adds routed experts introduced in #28284 to openai entrypoint

Test Plan

new unit tests

Test Result

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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.
• (Optional) Release notes update. If your change is user facing, please update the release notes draft in the Google Doc.

[gemini-code-assist]

Code Review

This pull request introduces the functionality to return routed expert indices in OpenAI-compatible chat and completion responses. It adds a routed_experts field to the response protocols and updates the serving logic to populate this field from the model output when the --enable-return-routed-experts flag is enabled. Additionally, a new test suite is included to verify the correct shape and values of the returned expert data. I have no feedback to provide.

[SumanthRH]

What is really needed here is support in the tokens-in-tokens-out /inference/v1/generate endpoint.

Can you replicate the modifications here:

vllm/vllm/entrypoints/serve/disagg/serving.py

Line 46 in 4b506ff

class ServingTokens(OpenAIServing):

vllm/vllm/entrypoints/serve/disagg/protocol.py

Line 117 in 81994e1

class GenerateResponseChoice(BaseModel):

and add a test?

[SumanthRH]

LGTM