added test case for test_rds_instance_backup_enabled

[Khushi-Kalantri-Comprinno]

Context

This change enhances the test coverage of the rds_instance_deletion_protection check.

Description

• Rewrote the test case test_rds_instance_deletion_protection.py to use CheckMetadata instead of a plain dictionary.
• Added new test for missing DeletionProtection key scenario.
• Added exception handling test for ClientError.
• Updated assertion to match the actual summary returned by the check implementation.
• Ensured the test structure follows the same format as the reference IAM password policy test.

Checklist

• Added new checks? If yes, reviewed necessary permissions
• Code covered by tests (if not, explain why)
• Documentation follows Google Python Style Guide
• Considered if backporting is needed

License

I confirm that my contribution is made under the terms of the Apache 2.0 license.