Fix #4270: Change default wire_api from Responses to Chat

[LIHUA919]

Resolves 'messages in request are illegal' 400 Bad Request error by changing the default OpenAI provider configuration to use the stable Chat Completions API instead of the experimental Responses API.

• Default OpenAI provider now uses WireApi::Chat instead of WireApi::Responses
• Maintains backward compatibility for explicit Responses API usage
• All HTTP client and message format tests pass (29/29 tests)

Fixes #4270

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