Use HttpCompletionOption.ResponseHeadersRead for faster response start

[GmausDev]

Summary

Use HttpCompletionOption.ResponseHeadersRead when making HTTP requests to start processing the response as soon as headers are received, rather than waiting for the entire response body.

Current Behavior

var response = await _httpClient.PostAsJsonAsync("chat/completions", request, ...);

The default HttpCompletionOption.ResponseContentRead waits for the entire response before returning.

Proposed Change

var response = await _httpClient.SendAsync(request, HttpCompletionOption.ResponseHeadersRead, cancellationToken);

Expected Benefits

• Faster response start - Begin processing immediately when headers arrive
• Better memory efficiency - Stream response directly instead of buffering
• Foundation for streaming - Required infrastructure for SSE/streaming support

Priority

🟡 P1 - High Impact

Technical Notes

• Need to ensure proper disposal of response stream
• Consider combining with ReadAsStreamAsync() for optimal memory usage
• This change is a prerequisite for implementing streaming responses