fix(openai): avoid developer transcript resets

- Narrow websocket transcript replacement detection to assistant outputs and function calls
- Preserve existing merge behavior for follow-up developer messages without previous_response_id
- Add a regression test covering mid-session developer message updates

sdk/api/handlers/openai/openai_responses_websocket.go

@@ -374,7 +374,7 @@ func shouldReplaceWebsocketTranscript(rawJSON []byte, nextInput gjson.Result) bo
 			return true
 		case "message":
 			role := strings.TrimSpace(item.Get("role").String())
-			if role == "assistant" || role == "developer" {
+			if role == "assistant" {
 				return true
 			}
 		}