fix(responses): handle response.done terminal events (#2142)

Treat response.done as a terminal OpenAI Responses stream event so
passthrough streams ending with response.done are not flagged incomplete
and no synthetic response.failed is emitted. Restore the data: [DONE]
sentinel for same-format Responses passthrough streams.

Co-authored-by: Cursor <cursoragent@cursor.com>

open-sse/utils/responsesStreamHelpers.js

@@ -5,6 +5,7 @@ import { formatSSE } from "./streamHelpers.js";
 // Responses API events that signal the stream has reached a terminal state
 const OPENAI_RESPONSES_TERMINAL_EVENTS = new Set([
   "response.completed",
+  "response.done",
   "response.failed",
   "error"
 ]);