fix(mail): align queue, outbound hostname, and DKIM selector behavior across the mail server APIs

ts/mail/interfaces.storage.ts

@@ -0,0 +1,13 @@
+export interface IStorageManagerLike {
+  get?(key: string): Promise<string | null>;
+  set?(key: string, value: string): Promise<void>;
+  list?(prefix: string): Promise<string[]>;
+  delete?(key: string): Promise<void>;
+}
+
+export function hasStorageManagerMethods<T extends keyof IStorageManagerLike>(
+  storageManager: IStorageManagerLike | undefined,
+  methods: T[],
+): storageManager is IStorageManagerLike & Required<Pick<IStorageManagerLike, T>> {
+  return !!storageManager && methods.every((method) => typeof storageManager[method] === 'function');
+}