pkg/webhook: test Multi-Cluster Ingress

In this commit we introduce unit tests for the `Validation Admission`. These
tests ensure correct behavior for various validation scenarios and improve coverage:

- Tests the behavior when decoding the request object fails, verifying
  that admission is denied with an appropriate error message.
- Validates that the webhook denies admission when encountering invalid values in the
  `MultiClusterIngress` spec, ensuring error messaging.
- Confirms that valid `MultiClusterIngress` objects are admitted without errors.