refactor: centralize target errors and cache lookups

src/infra/outbound/target-errors.ts

@@ -0,0 +1,8 @@
+export function missingTargetMessage(provider: string, hint?: string): string {
+  const suffix = hint ? ` ${hint}` : "";
+  return `Delivering to ${provider} requires target${suffix}`;
+}
+
+export function missingTargetError(provider: string, hint?: string): Error {
+  return new Error(missingTargetMessage(provider, hint));
+}