fix(core): backfill OpenAI responses model in spans

[meronogbai]

Summary

OpenAI Responses API calls can omit model in the request payload when using stored prompts. In that case, Sentry currently records gen_ai.request.model = "unknown" even though the final OpenAI response includes the actual model.

This change keeps the existing request-time fallback behavior, but overwrites the request model from response.model once the Responses API response arrives. It also updates the span name from chat unknown to chat .

This is now handled for both:

• non-streaming Responses API calls
• streaming Responses API calls

Agents dashboard screenshots

In production where this change isn't applied:

In development where this change was applied manually:

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Closes #issue_link_here

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