[Bugfix] Replace assert with ValueError for response_format validatio…

[antonovsergey93]

Purpose

When the /v1/chat/completions endpoint receives a request with response_format type json_schema but without the required json_schema field, the server crashes with an AssertionError, resulting in a 500 Internal Server Error.

Fixes #35438

This is the same class of issue addressed in #35456 for the /v1/completions endpoint

Test Plan

pytest tests/entrypoints/openai/test_chat_error.py -v

Test Result

Testing started at 14:58 ...
Launching pytest with arguments test_chat_error.py::test_json_schema_response_format_missing_schema --no-header --no-summary -q in /Users/santonov/github/vllm/tests/entrypoints/openai

============================= test session starts ==============================
collecting ... collected 1 item

test_chat_error.py::test_json_schema_response_format_missing_schema PASSED [100%]

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• The purpose of the PR, such as "Fix some issue (link existing issues this PR will resolve)".
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[gemini-code-assist]

Code Review

The code changes include adding a test case to check for a validation error when the json_schema field is missing in the response_format and adding a validator to the ChatCompletionRequest class to ensure that when the response_format type is 'json_schema', the 'json_schema' field is provided. The reviewer commented that the assert statement should be replaced with a more appropriate error handling mechanism, such as raising a ValueError or a custom exception, to provide more informative error messages and prevent the server from crashing.

[gemini-code-assist]

The assert statement should be replaced with a more appropriate error handling mechanism, such as raising a ValueError or a custom exception, to provide more informative error messages and prevent the server from crashing. This is especially important in production environments.

                raise ValueError("structural_tag cannot be None when response_format type is 'structural_tag'")

[DarkLight1337]

Sorry, #35510 came first