Update ObjectExtensionsTests to Include Derived Types

[brminnick]

Description of Change

This PR adds Unit Tests to ObjectExtensionsTests to ensure derived types are supported by .Assign() and .Invoke().

Linked Issues

• Fixes [Bug] .Assign Extension does not work with inherited controls of Label and Entry #135
  • Technically, it doesn't "fix" the Issues, but rather confirms that we cannot reproduce the Issue

PR Checklist

• Has a linked Issue, and the Issue has been approved(bug) or Championed (feature/proposal)
• Has tests (if omitted, state reason in description)
• Has samples (if omitted, state reason in description)
• Rebased on top of main at time of PR
• Changes adhere to coding standard

Additional information

This PR does not change any code in the CommunityToolkit.Maui.Markup library.

This PR technically, it doesn't "fix" #135. Rather, it confirms that we cannot reproduce #135.

This PR also adds <WarningsAsErrors>CS1710</WarningsAsErrors> to avoid duplicate XML Comments

[bijington]

Looks good to me!