fix(go): avoid panic when http.DefaultTransport is wrapped

defaultHTTPClient performed an unchecked type assertion on
http.DefaultTransport, which panicked for any caller that had wrapped
the global transport (e.g. otelhttp.NewTransport for distributed
tracing). When the assertion fails, fall back to the wrapped
RoundTripper as-is — preserving the caller's wrapping at the cost of
ResponseHeaderTimeout, which is strictly better than panicking.

Author: stainless-app[bot]

default_http_client.go

@@ -14,11 +14,17 @@ import (
 const defaultResponseHeaderTimeout = 10 * time.Minute
 
 // defaultHTTPClient returns an [*http.Client] used when the caller does not
-// supply one via [option.WithHTTPClient]. It clones [http.DefaultTransport]
-// and adds a [http.Transport.ResponseHeaderTimeout] so stuck connections
-// fail fast instead of compounding across retries.
+// supply one via [option.WithHTTPClient]. When [http.DefaultTransport] is the
+// stdlib [*http.Transport], it is cloned and a [http.Transport.ResponseHeaderTimeout]
+// is set so stuck connections fail fast instead of compounding across retries.
+// If [http.DefaultTransport] has been wrapped (for example by otelhttp for
+// distributed tracing), the wrapping is preserved and the header timeout is
+// skipped.
 func defaultHTTPClient() *http.Client {
-	transport := http.DefaultTransport.(*http.Transport).Clone()
-	transport.ResponseHeaderTimeout = defaultResponseHeaderTimeout
-	return &http.Client{Transport: transport}
+	if t, ok := http.DefaultTransport.(*http.Transport); ok {
+		t = t.Clone()
+		t.ResponseHeaderTimeout = defaultResponseHeaderTimeout
+		return &http.Client{Transport: t}
+	}
+	return &http.Client{Transport: http.DefaultTransport}
 }