test(auth): tighten R25 + sign-in controller assertions per code review

Code-quality reviewer flagged:
- R25 'malformed output' tests used .rejects.toThrow(/parse|invalid/i)
  which could match unrelated errors. Replaced with
  .rejects.toBeInstanceOf(ZodError) for both sign-in and sign-up.
- sign-in controller test asserted result.name only as truthy; the
  parallel sign-up test was already precise (toBe('session')). Tightened
  to match.

61 tests still passing for @repo/auth.

packages/auth/src/interface-adapters/controllers/sign-in.controller.test.ts

@@ -20,8 +20,7 @@ describe("signInController", () => {
       username: "alice",
       password: "testpassword",
     });
-    expect(result).toBeDefined();
-    expect(result.name).toBeTruthy();
+    expect(result.name).toBe("session");
     expect(result.value).toBeTruthy();
   });