Add test coverage for registry venv installation and VSCode error han…

[suung]

…dling UI

• Add registry venv installation tests (venv usage, fallback to system pip, error handling)
• Add VSCode extension error handling UI flow tests (preserve installation status on runtime errors, flag detection failed only for 'not found' errors, custom command handling, error message display)
• All registry venv tests passing

Pull Request Checklist

Legal Requirements

• I have read and agree to the Contributor License Agreement
• I understand that by contributing to this project, I grant the Carbonara team a perpetual, irrevocable license to use my contributions under the BSD license, while the public project remains under AGPL-3.0-or-later

Code Quality

• My code follows the project's coding style and conventions
• I have added appropriate tests for my changes
• All new and existing tests pass
• I have updated documentation where necessary

Description

Type of Change

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)
• Documentation update
• Refactoring (no functional changes)
• Performance improvement
• Test improvements

Testing

• Unit tests
• Integration tests
• Manual testing
• E2E tests (if applicable)

Additional Notes