Discord: add PluralKit sender identity resolver (#5838)

* Discord: add PluralKit sender identity resolver

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

src/discord/pluralkit.ts

@@ -0,0 +1,58 @@
+import { resolveFetch } from "../infra/fetch.js";
+
+const PLURALKIT_API_BASE = "https://api.pluralkit.me/v2";
+
+export type DiscordPluralKitConfig = {
+  enabled?: boolean;
+  token?: string;
+};
+
+export type PluralKitSystemInfo = {
+  id: string;
+  name?: string | null;
+  tag?: string | null;
+};
+
+export type PluralKitMemberInfo = {
+  id: string;
+  name?: string | null;
+  display_name?: string | null;
+};
+
+export type PluralKitMessageInfo = {
+  id: string;
+  original?: string | null;
+  sender?: string | null;
+  system?: PluralKitSystemInfo | null;
+  member?: PluralKitMemberInfo | null;
+};
+
+export async function fetchPluralKitMessageInfo(params: {
+  messageId: string;
+  config?: DiscordPluralKitConfig;
+  fetcher?: typeof fetch;
+}): Promise<PluralKitMessageInfo | null> {
+  if (!params.config?.enabled) {
+    return null;
+  }
+  const fetchImpl = resolveFetch(params.fetcher);
+  if (!fetchImpl) {
+    return null;
+  }
+  const headers: Record<string, string> = {};
+  if (params.config.token?.trim()) {
+    headers.Authorization = params.config.token.trim();
+  }
+  const res = await fetchImpl(`${PLURALKIT_API_BASE}/messages/${params.messageId}`, {
+    headers,
+  });
+  if (res.status === 404) {
+    return null;
+  }
+  if (!res.ok) {
+    const text = await res.text().catch(() => "");
+    const detail = text.trim() ? `: ${text.trim()}` : "";
+    throw new Error(`PluralKit API failed (${res.status})${detail}`);
+  }
+  return (await res.json()) as PluralKitMessageInfo;
+}