[Bug]: make `test_openai_schema.py` pass, enable it in CI

[davidxia]

This is a follow up to PR #17664 and issues like #17037 and #17038.

As of the time of this writing, the only sub-test in tests/entrypoints/openai/test_openai_schema.py that consistently fails is POST /tokenize caused by

vllm/vllm/entrypoints/chat_utils.py

Line 1083 in 98ea356

raise NotImplementedError(f"Unknown part type: {part_type}")

when part_type is "file". The expected response is 200 as documented by the OpenAPI spec.

How should we make this test pass? This is a requirement to eventually enabling the test in CI by removing --ignore=entrypoints/openai/test_openai_schema.py in test-pipeline.yaml.

============================================================================================ FAILURES =============================================================================================
_____________________________________________________________________ test_openapi_stateless (verbose_name='POST /tokenize') ______________________________________________________________________

    @wraps(test)
>   def test_function(*args: Any, **kwargs: Any) -> Any:

.venv/lib/python3.12/site-packages/schemathesis/_hypothesis.py:83: 
_ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _

case = Case(body={'messages': [{'content': [{'file': {}, 'type': 'file'}], 'role': 'user'}]})

    @schema.parametrize(tag="debug")
    @schema.override(headers={"Content-Type": "application/json"})
    def test_openapi_stateless(case: schemathesis.Case):
        key = (
            case.operation.method.upper(),
            case.operation.path,
        )
        timeout = {
            ("POST", "/v1/chat/completions"): 99999,
        }.get(key, 10)
    
        #No need to verify SSL certificate for localhost
>       case.call_and_validate(verify=False, timeout=timeout)
E       schemathesis.exceptions.CheckFailed: 
E       
E       1. Server error
E       
E       [500] Internal Server Error:
E       
E           `{"object":"error","message":"Unknown part type: file","type":"Internal Server Error","param":null,"code":500}`
E       
E       Reproduce with: 
E       
E           curl -X POST -H 'Content-Type: application/json' -d '{"messages": [{"content": [{"file": {}, "type": "file"}], "role": "user"}]}' --insecure http://localhost:48767/tokenize
E       
E       Falsifying example: test_openapi_stateless(
E           case=,
E       )

tests/entrypoints/openai/test_openai_schema.py:57: CheckFailed

$ curl -X POST -H 'Content-Type: application/json' -d '{"messages": [{"content": [{"file": {}, "type": "file"}], "role": "user"}]}' --insecure http://localhost:8000/tokenize -v
Note: Unnecessary use of -X or --request, POST is already inferred.
* Host localhost:8000 was resolved.
* IPv6: ::1
* IPv4: 127.0.0.1
*   Trying [::1]:8000...
* connect to ::1 port 8000 from ::1 port 40420 failed: Connection refused
*   Trying 127.0.0.1:8000...
* Connected to localhost (127.0.0.1) port 8000
> POST /tokenize HTTP/1.1
> Host: localhost:8000
> User-Agent: curl/8.5.0
> Accept: */*
> Content-Type: application/json
> Content-Length: 75
> 
< HTTP/1.1 500 Internal Server Error
< date: Mon, 12 May 2025 17:32:11 GMT
< server: uvicorn
< content-length: 109
< content-type: application/json
< 
* Connection #0 to host localhost left intact
{"object":"error","message":"Unknown part type: file","type":"Internal Server Error","param":null,"code":500}

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