[FEAT] - Improve test coverage and refactor assertions with Verify

[dylanlangston]

Is your feature request related to a problem? Please describe.

The current test suite lacks comprehensive coverage in several areas, and existing test assertions could be more maintainable. Many assertions are overly specific, making tests brittle when implementation details change. This makes it harder to refactor code with confidence.

Describe the solution you'd like

Expand and improve the test coverage while refactoring assertions to be more generic and maintainable using the Verify library's snapshot testing approach:

• Review existing tests in AvaloniaXKCD.Tests
• Identify areas lacking coverage (especially core logic in XKCDCore, ExplainXKCDClient.cs)
• Add unit tests for untested functionality
• Leverage the https://github.com/VerifyTests/Verify Library
• Replace dedicated type assertions with the recommended approach of creating new VerifierSettings with build actions that run assertions on snapshot output. https://github.com/dylanlangston/Axkcd/blob/master/src/AvaloniaXKCD.Tests/Verify/Plugins/VerifyAssertionsPlugin.cs
• Improve test orchestration (parallelization, shared setup, CI integration)
• Update documentation on how to run and extend the test suite

Describe alternatives you've considered

No response

Additional context

Acceptance Criteria:

• Test suite passes on all target platforms (Windows, macOS, Linux)
• Assertions follow generic patterns using VerifierSettings
• Coverage metrics show measurable improvement over baseline
• Tests are easy to understand and extend by new contributors
• Documentation explains how to run tests and add new ones
• CI integration is improved with reporting