Refactor Migration.cleanup method for testability and add comprehensive unit tests

[coderabbitai]

Problem

The Migration.cleanup method in pkg/controller/plan/migration.go is currently difficult to unit test due to tight coupling with multiple dependencies:

• r.kubevirt (KubeVirt interface)
• r.builder (Builder interface)
• r.provider (Provider interface)
• r.destination (DestinationClient interface)

This makes it challenging to write effective unit tests that can verify the cleanup logic without complex integration test setup.

Context

This issue was identified during PR #2517 when implementing the DeleteVmOnFailMigration feature. The team decided to defer the refactoring effort to keep the feature implementation focused.

Backlinks:

• PR: MTV-3165 | Add deleteVmOnFailMigration field to plan #2517
• Discussion: MTV-3165 | Add deleteVmOnFailMigration field to plan #2517 (comment)

Tasks

Phase 1: Refactor for testability

• Extract cleanup logic into smaller, focused functions
• Implement dependency injection pattern for the cleanup method
• Consider using interfaces to abstract external dependencies
• Ensure backward compatibility with existing code

Phase 2: Add comprehensive unit tests

• Test VM deletion logic with DeleteVmOnFailMigration flag variations
• Test cleanup behavior for successful vs failed migrations
• Test error handling and propagation in cleanup operations
• Test volume populator support scenarios
• Test warm snapshot removal logic
• Achieve high test coverage on cleanup logic paths

Acceptance Criteria

• The cleanup method should be easily unit testable with mocked dependencies
• Comprehensive test suite covering all cleanup scenarios and edge cases
• No regressions in existing cleanup behavior
• Maintain clean, readable code structure

Additional Notes

Integration tests already exist in a separate repository, so this work focuses specifically on unit test coverage.