fix(mindspace): fail closed on invalid page data delivery

chat-agent-run-gate.mjs

@@ -31,6 +31,18 @@ export function shouldPromoteSessionIdToStreaming(chatState) {
   return chatState !== 'idle';
 }
 
+/**
+ * Only transport uncertainty may continue a run after submit fails. A
+ * deterministic gateway error already has a terminal outcome and must return
+ * the chat UI to an error/idle state instead of leaving a Stop button active.
+ *
+ * @param {number | undefined | null} status
+ * @returns {boolean}
+ */
+export function shouldKeepStreamingAfterRunError(status) {
+  return status === 0 || status === 409 || Number(status) >= 500;
+}
+
 /**
  * Agent-run POST tracks portal request_id; Goose SSE may emit a different chat_request_id.
  * Adopt the upstream id so Message/Finish events are not dropped in the UI.