Revert "Expose DispatcherExtensions so it could be used outside of Maui as well"

[jfversluis]

Reverts #30488

Fixes #31139

[Copilot]

Pull Request Overview

This is a revert commit that undoes the previous changes made in PR #30488, which exposed DispatcherExtensions for use outside of MAUI. The revert addresses issue #31139 by removing the public API exposure and moving the extension methods back to the Controls project.

• Removes public DispatcherExtensions methods from the Core project's public API
• Removes comprehensive unit tests for the reverted extension methods
• Moves some dispatcher extension methods back to the Controls project with internal scope

Reviewed Changes

Copilot reviewed 14 out of 14 changed files in this pull request and generated 1 comment.

Show a summary per file

File	Description
src/Core/tests/UnitTests/Dispatching/DispatcherTests.cs	Removes NSubstitute dependency and all unit tests for DispatcherExtensions methods
src/Core/src/PublicAPI/net-tizen/PublicAPI.Unshipped.txt	Removes public API entries for DispatcherExtensions methods
src/Core/src/Dispatching/DispatcherExtensions.cs	Removes all DispatchIfRequired extension method implementations
src/Controls/src/Core/DispatcherExtensions.cs	Adds back limited DispatchIfRequired methods with internal scope
Multiple platform files	Removes unused Microsoft.Maui.Dispatching imports

[Copilot]

This revert removes public DispatcherExtensions methods that were previously available in the public API. This represents a breaking change that should not be included in minor versions or service releases.