fix(discord): default standalone threads to public type (#14147)

When creating a Discord thread without a messageId (standalone thread),
the Discord API defaults to type 12 (private). Most users expect public.

- Default standalone non-forum threads to ChannelType.PublicThread (11)
- Add optional type field to DiscordThreadCreate for explicit control

Closes #14147

src/discord/send.creates-thread.test.ts

@@ -114,7 +114,9 @@ describe("sendMessageDiscord", () => {
     await createThreadDiscord("chan1", { name: "thread" }, { rest, token: "t" });
     expect(postMock).toHaveBeenCalledWith(
       Routes.threads("chan1"),
-      expect.objectContaining({ body: { name: "thread" } }),
+      expect.objectContaining({
+        body: expect.objectContaining({ name: "thread", type: ChannelType.PublicThread }),
+      }),
     );
   });
 

src/discord/send.types.ts

@@ -72,6 +72,8 @@ export type DiscordThreadCreate = {
   name: string;
   autoArchiveMinutes?: number;
   content?: string;
+  /** Discord thread type (default: PublicThread for standalone threads). */
+  type?: number;
 };
 
 export type DiscordThreadList = {