[Frontend] Name the served models in the model-not-found 404

[TyroneNel]

Purpose

A wrong model name returns this body:

{"error":{"message":"The model `x` does not exist.","type":"NotFoundError","param":"model","code":404}}

The body does not say which names the server accepts. The reader must call /v1/models or open the server log. self.models.base_model_paths already holds that list, and /v1/models shows it. So the message can include it:

The model `x` does not exist. Served models: my-model.

--served-model-name makes this error easy to get: the served name is different from --model. A client that asks for the wrong name reads the current 404 as a missing route (see #58024). The status code, the error type and param do not change.

Test Plan

• Manual, against a v0.28.0 server: request an unknown model name and read the body. The "after" server runs v0.28.0 with this change applied as a patch (the same code change).
• pre-commit run on the changed file.

Test Result

Before:

$ curl -s -XPOST localhost:8000/tokenize -d '{"model":"not-a-served-model","prompt":"x"}'
{"error":{"message":"The model `not-a-served-model` does not exist.","type":"NotFoundError","param":"model","code":404}}

After (live server, --served-model-name qwen3.8-27b):

POST /tokenize model=not-a-served-model -> 404
{"error":{"message":"The model `not-a-served-model` does not exist. Served models: qwen3.8-27b.","type":"NotFoundError","param":"model","code":404}}

pre-commit: 13 hooks passed, 0 failed

If you want the message pinned by a test under tests/entrypoints/, tell me and I will add one.

[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.

[github-actions]

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[TyroneNel]

I pushed a small update:

• Rebased on current main. No conflicts, and the change is the same.
• DCO. The commit now has a sign-off that matches the author, so the DCO check passes.
• pre-commit passes on the changed file (13 hooks).
• Format. ruff format joined one line of the change. The behavior is the same.
• Live output. The PR body now shows the "after" rows from a running server.

When you have time, could a maintainer add the ready label, so that CI can run? Thank you!

[TyroneNel]

Hi! I rebased this PR on current main (b721a4c70).

What changed:

• The branch is now on top of main. The rebase had no conflicts.
• The diff is the same as before. The patch ID did not change.
• The commit keeps its DCO sign-off.

What I checked:

1. ruff check (v0.14.0, the pre-commit pin) passes on vllm/entrypoints/serve/engine/serving.py.
2. ruff format --check passes on the same file.
3. The file compiles. main added a request-body log call at the start of the same method. This change does not touch that call.

Status of CI: The pre-run-check job fails. The cause is not the code. The job needs the ready or verified label on the PR.

Request: When you have time, please add the ready label. Then CI can run the full suite.

Thank you for your time!