test: add sign-cache command integration tests

- Add TestSignCacheCommand_Integration for end-to-end command validation
- Add TestSignCacheCommand_ErrorHandling for error scenario testing
- Add TestSignCacheCommand_EnvironmentValidation for environment setup
- Add TestSignCacheCommand_ConfigurationValidation for config validation
- Add TestSignCacheCommand_FileHandling for file operation testing

Provides comprehensive integration testing of the sign-cache command with
mock implementations for external dependencies. Tests cover successful
execution, error handling, environment validation, and file operations.

Co-authored-by: Ona <no-reply@ona.com>

Author: leodido