Fix ApplicationStateGuard filename - rename Class1.cs to ApplicationStateGuard.cs

[Copilot]

The ApplicationStateGuard class in the TestFramework project was incorrectly placed in a file named Class1.cs. This violates .NET naming conventions where the filename should match the class name.

Changes Made

• Renamed src/TestFramework/TestFramework/Internal/Class1.cs to ApplicationStateGuard.cs
• Added proper namespace declaration: Microsoft.VisualStudio.TestTools.UnitTesting
• Maintained the exact same class implementation and method signatures

Before

// File: Class1.cs (incorrect filename)
// No namespace declaration

internal static class ApplicationStateGuard
{
    public static UnreachableException Unreachable([CallerFilePath] string? path = null, [CallerLineNumber] int line = 0)
        => new($"This program location is thought to be unreachable. File='{path}' Line={line}");
}

After

// File: ApplicationStateGuard.cs (correct filename)
namespace Microsoft.VisualStudio.TestTools.UnitTesting;

internal static class ApplicationStateGuard
{
    public static UnreachableException Unreachable([CallerFilePath] string? path = null, [CallerLineNumber] int line = 0)
        => new($"This program location is thought to be unreachable. File='{path}' Line={line}");
}

This change brings the TestFramework's ApplicationStateGuard implementation in line with the naming conventions used by other ApplicationStateGuard classes throughout the codebase (in Platform, Analyzers, and Adapter projects). The existing usage in Assert.AreEqual.cs continues to work unchanged since both files are in the same namespace.

Fixes #6297.

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