Discord: add PluralKit sender identity resolver (#5838)

* Discord: add PluralKit sender identity resolver

* fix: resolve PluralKit sender identities (#5838) (thanks @thewilloftheshadow)

src/discord/pluralkit.test.ts

@@ -0,0 +1,67 @@
+import { describe, expect, it, vi } from "vitest";
+import { fetchPluralKitMessageInfo } from "./pluralkit.js";
+
+type MockResponse = {
+  status: number;
+  ok: boolean;
+  text: () => Promise<string>;
+  json: () => Promise<unknown>;
+};
+
+const buildResponse = (params: { status: number; body?: unknown }): MockResponse => {
+  const body = params.body;
+  const textPayload = typeof body === "string" ? body : body == null ? "" : JSON.stringify(body);
+  return {
+    status: params.status,
+    ok: params.status >= 200 && params.status < 300,
+    text: async () => textPayload,
+    json: async () => body ?? {},
+  };
+};
+
+describe("fetchPluralKitMessageInfo", () => {
+  it("returns null when disabled", async () => {
+    const fetcher = vi.fn();
+    const result = await fetchPluralKitMessageInfo({
+      messageId: "123",
+      config: { enabled: false },
+      fetcher: fetcher as unknown as typeof fetch,
+    });
+    expect(result).toBeNull();
+    expect(fetcher).not.toHaveBeenCalled();
+  });
+
+  it("returns null on 404", async () => {
+    const fetcher = vi.fn(async () => buildResponse({ status: 404 }));
+    const result = await fetchPluralKitMessageInfo({
+      messageId: "missing",
+      config: { enabled: true },
+      fetcher: fetcher as unknown as typeof fetch,
+    });
+    expect(result).toBeNull();
+  });
+
+  it("returns payload and sends token when configured", async () => {
+    let receivedHeaders: Record<string, string> | undefined;
+    const fetcher = vi.fn(async (_url: string, init?: RequestInit) => {
+      receivedHeaders = init?.headers as Record<string, string> | undefined;
+      return buildResponse({
+        status: 200,
+        body: {
+          id: "123",
+          member: { id: "mem_1", name: "Alex" },
+          system: { id: "sys_1", name: "System" },
+        },
+      });
+    });
+
+    const result = await fetchPluralKitMessageInfo({
+      messageId: "123",
+      config: { enabled: true, token: "pk_test" },
+      fetcher: fetcher as unknown as typeof fetch,
+    });
+
+    expect(result?.member?.id).toBe("mem_1");
+    expect(receivedHeaders?.Authorization).toBe("pk_test");
+  });
+});