Add proxy support for OpenAI Responses API endpoint

[Copilot]

vLLM added support for the Responses API (/v1/responses) in PR #20504. This endpoint provides enhanced agentic capabilities but was not proxied by KubeAI.

Changes

• Route registration (internal/openaiserver/handler.go): Added /openai/v1/responses endpoint handler
• Request parsing (internal/apiutils/request.go): Added /v1/responses path case, reusing ChatCompletionRequest structure (matches vLLM's implementation)
• Test coverage (internal/modelproxy/handler_test.go): Added TestHandlerResponsesAPI to verify routing, model lookup, and request proxying

The Responses API follows the same request format as Chat Completions, so existing proxy logic (model routing, load balancing, retries, adapter handling) applies without modification.

// Request to /openai/v1/responses now properly routes through KubeAI
handle("/openai/v1/responses", http.StripPrefix("/openai", modelProxy))

Original prompt

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This section details on the original issue you should resolve

<issue_title>Responses API Support</issue_title>
<issue_description>vLLM recently added support for the new Responses API, which provides more flexible and agentic capabilities compared to the older endpoints. To take advantage of this improvement, KubeAI should add proxy support for the Responses API endpoint. This would allow users to access the new functionality seamlessly through KubeAI, enabling more advanced and interactive workflows. Adding this support ensures that users can leverage the enhanced features of vLLM’s Responses API.
vllm-project/vllm#20504

Relevant section in KubeAI router:
https://github.com/ErwinsExpertise/kubeai/blob/main/internal/openaiserver/handler.go#L38</issue_description>

Comments on the Issue (you are @copilot in this section)

• Fixes Responses API Support #1

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