feature!: improve Operation response classes

[cuongph87]

Types of changes

Implements #118:

• Adds comprehensive XML documentation for all public properties
• Applies required modifier to mandatory properties deserialized from JSON
• Properly marks nullable properties with ? for optional fields
• Changes properties from { get; set; } to { get; init; } for immutability
• Updates outdated JSON test files and corresponding tests
• Adds missing tests for better coverage

• 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 change)
• I have read the CONTRIBUTING document.
• My code follows the code style of this project.
• My change requires a change to the documentation.
• I have updated the documentation accordingly.
• I have added tests to cover my changes.
• All new and existing tests passed.