fix: preserve error cause, fix test mock typing

- Add { cause: err } to thrown Error for upstream debugging
- Use 'as unknown as string' cast for undefined-id test mock

extensions/discord/src/monitor/provider.test.ts

@@ -646,7 +646,7 @@ describe("monitorDiscordProvider", () => {
     const { monitorDiscordProvider } = await import("./provider.js");
     const runtime = baseRuntime();
 
-    clientFetchUserMock.mockResolvedValueOnce({ id: undefined, username: "NoId" });
+    clientFetchUserMock.mockResolvedValueOnce({ id: undefined as unknown as string, username: "NoId" });
 
     await expect(
       monitorDiscordProvider({

extensions/discord/src/monitor/provider.ts

@@ -884,7 +884,7 @@ export async function monitorDiscordProvider(opts: MonitorDiscordOpts = {}) {
       // messages pass through), self-message filtering is disabled (risk of
       // self-reply loops), and reply detection is broken. Let auto-restart
       // retry instead of running in a degraded state. See #42219.
-      throw new Error(`discord: cannot start without bot identity: ${String(err)}`);
+      throw new Error(`discord: cannot start without bot identity: ${String(err)}`, { cause: err });
     }
     if (!botUserId) {
       // fetchUser succeeded but returned no id — equally unsafe to continue.