docs(plan-1): note ThrowsAny for tamper test (.NET 9 AesGcm behavior)

docs/superpowers/plans/2026-08-28-plan-1-backend-foundation.md

@@ -1490,7 +1490,9 @@ public class EncryptionServiceTests
         var bytes = Convert.FromBase64String(wire);
         bytes[^1] ^= 0x01;          // flip a bit in the tag
         var tampered = Convert.ToBase64String(bytes);
-        Assert.Throws<CryptographicException>(() => svc.Decrypt(tampered));
+        // ThrowsAny (not Throws) because .NET 9 throws the more specific
+        // AuthenticationTagMismatchException subclass on tamper.
+        Assert.ThrowsAny<CryptographicException>(() => svc.Decrypt(tampered));
     }
 
     [Fact]