Fixed issue when view is hidden and then re-shown

[Paul Dumais (pauldumais)]

This pull request improves how RdpClientView manages the lifecycle of its native control handle, especially when the control is temporarily detached (such as during docking or tab operations). The changes ensure that the native control (HWND) is not destroyed prematurely and is properly released when the view is disposed. The tests are updated to verify this new behavior.

Improvements to native control lifecycle management:

• Added a detachedNativeControl field to RdpClientView to retain the HWND when the control is detached but not disposed, preventing premature destruction during docking or tab detachment scenarios.
• Modified DestroyNativeControlCore to retain the native control in detachedNativeControl if the view is not disposed, and only destroy it when the view is explicitly disposed.
• Updated CreateNativeControlCore to reuse the retained detachedNativeControl when reattaching, ensuring the existing session is preserved instead of creating a new one.
• Updated Dispose to explicitly destroy the retained native control if it exists, ensuring proper cleanup when the view is disposed.

Test coverage enhancements:

• Added tests in AvaloniaHosting.cs to verify that RdpClientView properly reuses a detached native control and correctly destroys it upon disposal, preventing resource leaks or premature destruction.