fix(go/fio): nil http client panic in fio.New

When token is empty, New falls back to transparentClient with the
caller-supplied hc. main.go passes nil, so the first Do() call panicked.
Default to http.DefaultClient when hc is nil.

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

go/internal/io/fio/client.go

@@ -4,6 +4,7 @@ package fio
 
 import (
 	"context"
+	"net/http"
 	"time"
 )
 
@@ -29,7 +30,11 @@ type Client interface {
 }
 
 // New returns an apiClient when token is non-empty, otherwise a transparentClient.
+// hc may be nil, in which case http.DefaultClient is used.
 func New(token, accountNum string, hc httpDoer) Client {
+	if hc == nil {
+		hc = http.DefaultClient
+	}
 	if token != "" {
 		return &apiClient{token: token, hc: hc}
 	}