chore: request unit test coverage

[jopmiddelkamp]

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 several request builder classes in the SDK. The primary focus is testing URI construction, parameter handling, and exception scenarios.

Key changes:

• Added new test coverage for PathsRequestBuilder (entirely new test file)
• Extended test coverage for TransactionsRequestBuilder, OperationsRequestBuilder, and EffectsRequestBuilder
• Tests cover URI building, parameter validation, null/empty argument handling, and data deserialization

Reviewed changes

Copilot reviewed 4 out of 4 changed files in this pull request and generated 8 comments.

File	Description
TransactionsRequestBuilderTest.cs	Added tests for Transaction() method with URI/ID, null parameter validation, and IncludeFailed() with false value
PathsRequestBuilderTest.cs	New comprehensive test file covering all PathsRequestBuilder methods including URI construction, parameter handling, and Execute()
OperationsRequestBuilderTest.cs	Added tests for Operation() method, extensive null/empty/invalid parameter validation, and IncludeFailed() with false value
EffectsRequestBuilderTest.cs	Added null parameter validation tests and liquidity pool URI construction tests with both LiquidityPoolId and string ID

[cuongph87]

Changes requested.