Update SK samples and tests

[rogerbarreto]

Semantic Kernel now supports natively the Microsoft.Extensions.AI types and this pull request refactors the SemanticKernel examples and documentation reflecting that change moving forward as the ideal approach.

Refactoring and Test Improvements:

• Replaced IChatCompletionService with IChatClient in TestSKInit, simplifying the service creation process and aligning with modern patterns. Updated the test logic to reflect this change.

Plugin Optimization:

• Updated the GetWeather method in the SkPlugins class to return a completed Task directly, improving performance by avoiding unnecessary async/await overhead.

Documentation Alignment:

• Updated the example in the README.md to use the new IChatClient creation pattern, ensuring consistency with the refactored code.

[tghamm]

Thanks @rogerbarreto !