fix: preserve explicit zero values in native relay requests

dto/audio.go

@@ -15,7 +15,7 @@ type AudioRequest struct {
 	Voice          string          `json:"voice"`
 	Instructions   string          `json:"instructions,omitempty"`
 	ResponseFormat string          `json:"response_format,omitempty"`
-	Speed          float64         `json:"speed,omitempty"`
+	Speed          *float64        `json:"speed,omitempty"`
 	StreamFormat   string          `json:"stream_format,omitempty"`
 	Metadata       json.RawMessage `json:"metadata,omitempty"`
 }