fix: enable runtime fallback for delegated child sessions (#2357)

src/hooks/runtime-fallback/last-user-retry-parts.ts

@@ -0,0 +1,25 @@
+type SessionMessagesResponse = {
+  data?: Array<{
+    info?: Record<string, unknown>
+    parts?: Array<{ type?: string; text?: string }>
+  }>
+}
+
+export function getLastUserRetryParts(
+  messagesResponse: unknown,
+): Array<{ type: "text"; text: string }> {
+  const messages = (messagesResponse as SessionMessagesResponse).data
+  const lastUserMessage = messages?.filter((message) => message.info?.role === "user").pop()
+  const lastUserParts =
+    lastUserMessage?.parts
+    ?? (lastUserMessage?.info?.parts as Array<{ type?: string; text?: string }> | undefined)
+
+  return (lastUserParts ?? [])
+    .filter(
+      (part): part is { type: "text"; text: string } =>
+        part.type === "text"
+        && typeof part.text === "string"
+        && part.text.length > 0,
+    )
+    .map((part) => ({ type: "text" as const, text: part.text }))
+}