Use UnsafeAccessor in System.Diagnostics.Debug.Tests

[jkoritzinsky]

Fixes #120722

[Copilot]

Pull Request Overview

This PR modernizes the System.Diagnostics.Debug.Tests by replacing reflection-based access to internal Debug and DebugProvider members with the more efficient and type-safe UnsafeAccessor pattern. This addresses issue #120722 and follows modern .NET best practices for accessing private/internal members in tests.

Key changes:

• Replaces reflection calls with UnsafeAccessor methods for accessing internal Debug provider functionality
• Adds a new public GetProvider() method to Debug class to support testing needs
• Updates compatibility suppressions to handle the new public API

Reviewed Changes

Copilot reviewed 4 out of 4 changed files in this pull request and generated 4 comments.

File	Description
src/libraries/System.Runtime/tests/System.Diagnostics.Debug.Tests/DebugTests.cs	Replaces reflection-based field access with UnsafeAccessor methods for cleaner, more efficient test code
src/libraries/System.Private.CoreLib/src/System/Diagnostics/Debug.cs	Adds public GetProvider() method to expose the internal provider for testing
src/libraries/System.Private.CoreLib/ref/System.Private.CoreLib.ExtraApis.txt	Registers the new GetProvider method in the extra APIs list
src/coreclr/System.Private.CoreLib/CompatibilitySuppressions.xml	Adds compatibility suppression for the new GetProvider method

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[jkotas]

Cleanup this as well

runtime/src/libraries/System.Runtime/tests/System.Diagnostics.Debug.Tests/System.Diagnostics.Debug.Tests.csproj

Lines 6 to 13 in 0327b0d

	<!-- Some tests need types like System.Diagnostics.DebugProvider which are only exposed from System.Private.CoreLib -->
	<CompileUsingReferenceAssemblies>false</CompileUsingReferenceAssemblies>
	<!-- Active issue: https://github.com/dotnet/runtime/issues/87740 -->
	<ShouldILStrip>false</ShouldILStrip>
	</PropertyGroup>
	<ItemGroup>
	<DefaultReferenceExclusion Include="System.Collections" />
	<ProjectReference Include="$(CoreLibProject)" />

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