chore: operations unit test coverage

[jopmiddelkamp]

Summary

Types of changes

• 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.

[Copilot]

Pull request overview

This PR adds comprehensive unit test coverage for the ChangeTrustOperation class, testing constructor behavior, XDR serialization/deserialization, and round-trip scenarios for various asset types (native, CreditAlphaNum4, CreditAlphaNum12, and liquidity pool shares).

Key changes:

• Adds 30+ test methods covering all public constructors and methods of ChangeTrustOperation
• Tests edge cases like null parameters, zero limits, max limits, and unknown asset types
• Validates XDR serialization/deserialization for all supported asset types
• Tests both current and obsolete constructor overloads