fix(openai): drain error response body for HTTP connection reuse

[HUQIANTAO]

Summary

After reading the limited (4KB) error body from a non-200 response, drain any remaining body to io.Discard before closing. This ensures the HTTP/2 connection can be reused by Go's transport pool.

Before

msg, _ := io.ReadAll(io.LimitReader(resp.Body, 4096))
resp.Body.Close()

After

msg, _ := io.ReadAll(io.LimitReader(resp.Body, 4096))
_, _ = io.Copy(io.Discard, resp.Body)
resp.Body.Close()

Motivation

Go's http.Transport reuses connections only when the response body is fully drained. Without draining, each retry in sendWithRetry opens a new TCP connection instead of reusing the existing one. This is especially impactful for the retry loop (up to 3 attempts on transient errors).