fix: preserve interaction replies across chat recovery

src/chat/response-inputs.ts

@@ -1,5 +1,34 @@
 export type InputDelivery = "steer" | "queue";
 export type ResponseInput = { id: string; text: string; delivery: InputDelivery; status: "pending" | "delivered" | "cancelled" };
+type WaitingInboxRuntime = {
+  listPendingUserInputs: (sessionId: string) => Promise<ResponseInput[]>;
+  agentMessageState: (sessionId: string, id: string) => Promise<"missing" | "enqueued" | "delivered">;
+  enqueueUserInput: (sessionId: string, input: ResponseInput, options?: { resume?: boolean }) => Promise<void>;
+};
+
+/** A question boundary admits input without starting an unowned model loop. */
+export class WaitingResponseInputQueue {
+  private readonly operations = new Map<string, Promise<ResponseInput>>();
+  constructor(private readonly runtime: WaitingInboxRuntime) {}
+
+  async submit(sessionId: string, input: ResponseInput): Promise<ResponseInput> {
+    const operation = (this.operations.get(sessionId) ?? Promise.resolve()).catch(() => undefined).then(async () => {
+      const pending = await this.runtime.listPendingUserInputs(sessionId);
+      const previous = pending.find((item) => item.id === input.id);
+      if (previous) {
+        if (previous.text !== input.text) throw Object.assign(new Error("Input ID already belongs to another message"), { status: 409 });
+        return previous;
+      }
+      if (await this.runtime.agentMessageState(sessionId, input.id) === "delivered") return { ...input, status: "delivered" as const };
+      if (pending.length >= 20) throw Object.assign(new Error("At most 20 user messages may wait in this session"), { status: 429 });
+      await this.runtime.enqueueUserInput(sessionId, input, { resume: false });
+      return { ...input };
+    });
+    this.operations.set(sessionId, operation);
+    try { return await operation; }
+    finally { if (this.operations.get(sessionId) === operation) this.operations.delete(sessionId); }
+  }
+}
 type InboxRuntime = {
   enqueueUserInput: (sessionId: string, input: ResponseInput) => Promise<void>;
   listPendingUserInputs: (sessionId: string) => Promise<ResponseInput[]>;