feat: exit() accepts result value, waitUntilExit resolves with it

Change exit(error?: Error) to exit(errorOrResult?: unknown). When an
Error is passed, waitUntilExit rejects; otherwise it resolves with the
value. This matches Ink's behavior where exit("hello") resolves the
promise with "hello".

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

packages/testing/src/render.ts

@@ -31,7 +31,7 @@ export interface RenderResult {
   };
   terminal: Terminal;
   unmount(this: void): void;
-  waitUntilExit(this: void): Promise<void>;
+  waitUntilExit(this: void): Promise<unknown>;
 }
 
 function trimFrame(raw: string): string {