discord: auto-create thread when sending to Forum/Media channels (#12380)

* discord: auto-create thread when sending to Forum/Media channels

* Discord: harden forum thread sends (#12380) (thanks @magendary)

* fix: clean up discord send exports (#12380) (thanks @magendary)

---------

Co-authored-by: Shadow <shadow@clawd.bot>

src/discord/send.sends-basic-channel-messages.test.ts

@@ -1,4 +1,4 @@
-import { PermissionFlagsBits, Routes } from "discord-api-types/v10";
+import { ChannelType, PermissionFlagsBits, Routes } from "discord-api-types/v10";
 import { beforeEach, describe, expect, it, vi } from "vitest";
 import {
   deleteMessageDiscord,
@@ -58,7 +58,9 @@ describe("sendMessageDiscord", () => {
   });
 
   it("sends basic channel messages", async () => {
-    const { rest, postMock } = makeRest();
+    const { rest, postMock, getMock } = makeRest();
+    // Channel type lookup returns a normal text channel (not a forum).
+    getMock.mockResolvedValueOnce({ type: ChannelType.GuildText });
     postMock.mockResolvedValue({
       id: "msg1",
       channel_id: "789",
@@ -74,6 +76,89 @@ describe("sendMessageDiscord", () => {
     );
   });
 
+  it("auto-creates a forum thread when target is a Forum channel", async () => {
+    const { rest, postMock, getMock } = makeRest();
+    // Channel type lookup returns a Forum channel.
+    getMock.mockResolvedValueOnce({ type: ChannelType.GuildForum });
+    postMock.mockResolvedValue({
+      id: "thread1",
+      message: { id: "starter1", channel_id: "thread1" },
+    });
+    const res = await sendMessageDiscord("channel:forum1", "Discussion topic\nBody of the post", {
+      rest,
+      token: "t",
+    });
+    expect(res).toEqual({ messageId: "starter1", channelId: "thread1" });
+    // Should POST to threads route, not channelMessages.
+    expect(postMock).toHaveBeenCalledWith(
+      Routes.threads("forum1"),
+      expect.objectContaining({
+        body: {
+          name: "Discussion topic",
+          message: { content: "Discussion topic\nBody of the post" },
+        },
+      }),
+    );
+  });
+
+  it("posts media as a follow-up message in forum channels", async () => {
+    const { rest, postMock, getMock } = makeRest();
+    getMock.mockResolvedValueOnce({ type: ChannelType.GuildForum });
+    postMock
+      .mockResolvedValueOnce({
+        id: "thread1",
+        message: { id: "starter1", channel_id: "thread1" },
+      })
+      .mockResolvedValueOnce({ id: "media1", channel_id: "thread1" });
+    const res = await sendMessageDiscord("channel:forum1", "Topic", {
+      rest,
+      token: "t",
+      mediaUrl: "file:///tmp/photo.jpg",
+    });
+    expect(res).toEqual({ messageId: "starter1", channelId: "thread1" });
+    expect(postMock).toHaveBeenNthCalledWith(
+      1,
+      Routes.threads("forum1"),
+      expect.objectContaining({
+        body: {
+          name: "Topic",
+          message: { content: "Topic" },
+        },
+      }),
+    );
+    expect(postMock).toHaveBeenNthCalledWith(
+      2,
+      Routes.channelMessages("thread1"),
+      expect.objectContaining({
+        body: expect.objectContaining({
+          files: [expect.objectContaining({ name: "photo.jpg" })],
+        }),
+      }),
+    );
+  });
+
+  it("chunks long forum posts into follow-up messages", async () => {
+    const { rest, postMock, getMock } = makeRest();
+    getMock.mockResolvedValueOnce({ type: ChannelType.GuildForum });
+    postMock
+      .mockResolvedValueOnce({
+        id: "thread1",
+        message: { id: "starter1", channel_id: "thread1" },
+      })
+      .mockResolvedValueOnce({ id: "msg2", channel_id: "thread1" });
+    const longText = "a".repeat(2001);
+    await sendMessageDiscord("channel:forum1", longText, {
+      rest,
+      token: "t",
+    });
+    const firstBody = postMock.mock.calls[0]?.[1]?.body as {
+      message?: { content?: string };
+    };
+    const secondBody = postMock.mock.calls[1]?.[1]?.body as { content?: string };
+    expect(firstBody?.message?.content).toHaveLength(2000);
+    expect(secondBody?.content).toBe("a");
+  });
+
   it("starts DM when recipient is a user", async () => {
     const { rest, postMock } = makeRest();
     postMock
@@ -118,6 +203,7 @@ describe("sendMessageDiscord", () => {
     });
     postMock.mockRejectedValueOnce(apiError);
     getMock
+      .mockResolvedValueOnce({ type: ChannelType.GuildText })
       .mockResolvedValueOnce({
         id: "789",
         guild_id: "guild1",