Improve contravariance example in "Using Variance in Delegates" documentation

[Copilot]

The existing contravariance example in the "Using Variance in Delegates" documentation was causing confusion for users who couldn't understand how the MultiHandler method could handle both KeyDown and MouseClick events.

Changes Made

1. Created Complete Working Example

• Added a full console application in snippets/using-variance-in-delegates/ContravarianceExample.cs
• Demonstrates contravariance with custom KeyEventArgs and MouseEventArgs classes
• Shows how one MultiHandler method handles multiple event types
• Includes executable code that clearly demonstrates the concept

2. Enhanced Documentation

• Added "How contravariance works in this example" section with step-by-step explanation
• Provided numbered steps showing the logical flow of contravariance
• Explained why the compiler allows a method with EventArgs parameter to handle events expecting more specific types
• Added practical demonstration output description

3. Improved Code Organization

• Moved from inline code blocks to proper snippet references following documentation guidelines
• Created dedicated project file for the example
• Added comprehensive comments explaining the inheritance hierarchy and contravariance mechanics

Example Output

The new example produces clear output demonstrating the concept:

Demonstrating contravariance in event handlers:
Same MultiHandler method handles both events!

Simulating key press: Enter
MultiHandler called at: 18:08:30.172
  - Key event: Enter
Simulating mouse click at (100, 200)
MultiHandler called at: 18:08:30.180
  - Mouse event: (100, 200)

This directly addresses the user's confusion by showing exactly how contravariance enables one handler to work with multiple event types, making the documentation much clearer and more educational.

Fixes #18540.

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Internal previews

📄 File	🔗 Preview link
docs/csharp/programming-guide/concepts/covariance-contravariance/using-variance-in-delegates.md	docs/csharp/programming-guide/concepts/covariance-contravariance/using-variance-in-delegates

[BillWagner]

This is a great start. I have a few suggestions for improvement.

[BillWagner]

This is ready for final review.