Disallow RID on solution for MTP dotnet test

[Youssef1313]

Fixes #50147

FYI @baronfel

[github-actions]

This PR is targeting main, which is now for .NET 11-facing work. If you intended to target .NET 10, either retarget this PR to release/10.0.1xx or make sure you backport the change to release/10.0.1xx after merging. See #50394 for more details.

[Copilot]

Pull Request Overview

This PR fixes issue #50147 by preventing the use of RuntimeIdentifier (RID) specifications at the solution level when using MTP (Multi-Targeting Platform) dotnet test commands, which is not supported and should produce an error.

• Adds _MTPBuild target to existing MSBuild checks for solution-level RID usage
• Renames existing test method to be more specific about testing project-level arch options
• Adds new test to verify that using arch option on solution-level operations produces the expected error

Reviewed Changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated 1 comment.

File	Description
Microsoft.NET.Sdk.Solution.targets	Extends existing solution-level checks to include MTP build scenarios
GivenDotnetTestBuildsAndRunsTestsWithDifferentOptions.cs	Renames existing test and adds new test for solution-level arch option error handling