fix(followup): fall back to dispatcher when same-channel origin routing fails (#26109)

* fix(followup): fall back to dispatcher when same-channel origin routing fails

When routeReply fails for an originating channel that matches the
session's messageProvider, the onBlockReply callback was created by
that same channel's handler and can safely deliver the reply.
Previously the payload was silently dropped on any routeReply failure,
causing Feishu DM replies to never reach the user.

Cross-channel fallback (origin ≠ provider) still drops the payload to
preserve origin isolation.

Closes #25767

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

* fix: allow same-channel followup fallback routing (#26109) (thanks @Sid-Qin)

---------

Co-authored-by: Cursor <cursoragent@cursor.com>
Co-authored-by: Peter Steinberger <steipete@gmail.com>

src/auto-reply/reply/followup-runner.test.ts

@@ -8,6 +8,7 @@ import { createMockTypingController } from "./test-helpers.js";
 
 const runEmbeddedPiAgentMock = vi.fn();
 const routeReplyMock = vi.fn();
+const isRoutableChannelMock = vi.fn();
 
 vi.mock(
   "../../agents/model-fallback.js",
@@ -22,15 +23,30 @@ vi.mock("./route-reply.js", async (importOriginal) => {
   const actual = await importOriginal<typeof import("./route-reply.js")>();
   return {
     ...actual,
+    isRoutableChannel: (...args: unknown[]) => isRoutableChannelMock(...args),
     routeReply: (...args: unknown[]) => routeReplyMock(...args),
   };
 });
 
 import { createFollowupRunner } from "./followup-runner.js";
 
+const ROUTABLE_TEST_CHANNELS = new Set([
+  "telegram",
+  "slack",
+  "discord",
+  "signal",
+  "imessage",
+  "whatsapp",
+  "feishu",
+]);
+
 beforeEach(() => {
   routeReplyMock.mockReset();
   routeReplyMock.mockResolvedValue({ ok: true });
+  isRoutableChannelMock.mockReset();
+  isRoutableChannelMock.mockImplementation((ch: string | undefined) =>
+    Boolean(ch?.trim() && ROUTABLE_TEST_CHANNELS.has(ch.trim().toLowerCase())),
+  );
 });
 
 const baseQueuedRun = (messageProvider = "whatsapp"): FollowupRun =>
@@ -336,7 +352,7 @@ describe("createFollowupRunner messaging tool dedupe", () => {
     expect(store[sessionKey]?.outputTokens).toBe(50);
   });
 
-  it("does not fall back to dispatcher when explicit origin routing fails", async () => {
+  it("does not fall back to dispatcher when cross-channel origin routing fails", async () => {
     const onBlockReply = vi.fn(async () => {});
     runEmbeddedPiAgentMock.mockResolvedValueOnce({
       payloads: [{ text: "hello world!" }],
@@ -359,6 +375,30 @@ describe("createFollowupRunner messaging tool dedupe", () => {
     expect(onBlockReply).not.toHaveBeenCalled();
   });
 
+  it("falls back to dispatcher when same-channel origin routing fails", async () => {
+    const onBlockReply = vi.fn(async () => {});
+    runEmbeddedPiAgentMock.mockResolvedValueOnce({
+      payloads: [{ text: "hello world!" }],
+      meta: {},
+    });
+    routeReplyMock.mockResolvedValueOnce({
+      ok: false,
+      error: "outbound adapter unavailable",
+    });
+
+    const runner = createMessagingDedupeRunner(onBlockReply);
+
+    await runner({
+      ...baseQueuedRun(" Feishu "),
+      originatingChannel: "FEISHU",
+      originatingTo: "ou_abc123",
+    } as FollowupRun);
+
+    expect(routeReplyMock).toHaveBeenCalled();
+    expect(onBlockReply).toHaveBeenCalledTimes(1);
+    expect(onBlockReply).toHaveBeenCalledWith(expect.objectContaining({ text: "hello world!" }));
+  });
+
   it("routes followups with originating account/thread metadata", async () => {
     const onBlockReply = vi.fn(async () => {});
     runEmbeddedPiAgentMock.mockResolvedValueOnce({