[CI][Frontend] Return 422 instead of 500 for invalid Anthropic tool_choice

[AndreasKaratzas]

When tool_choice is {"type": "tool"} without a name, the /v1/messages endpoint crashes with a 500 Internal Server Error. This happens because _convert_anthropic_to_openai_request passes name=None to ChatCompletionNamedToolChoiceParam, which fails Pydantic validation as an unhandled exception.

The Anthropic API spec requires name when tool_choice.type is "tool". This PR adds a model_validator to AnthropicToolChoice so the constraint is enforced at request parsing time, producing a proper 422 validation error instead of a 500.

Also adds the /v1/messages endpoint to the longer timeout map in the schema test, since it is a completion endpoint.

Motivation

AMD nightly CI: https://buildkite.com/vllm/amd-ci/builds/4788/steps/canvas?sid=019c6019-8f6f-4b50-a415-c3f443c97c5e&tab=output
schemathesis property-based testing in test_openapi_stateless[POST /v1/messages].

[gemini-code-assist]

Code Review

This pull request correctly addresses a 500 Internal Server Error that occurred with invalid tool_choice parameters in the Anthropic API compatibility layer. The introduction of a model_validator in AnthropicToolChoice is a clean and effective way to enforce the API contract at the request parsing stage, properly returning a 422 validation error instead of crashing. The associated change to extend the timeout for the /v1/messages endpoint in the schema tests is also appropriate, aligning it with other completion endpoints. The changes are well-targeted and improve the robustness of the server. Good work.