This PR implements validation to show errors when VSTest-specific command-line switches are used with Microsoft Testing Platform (MTP), helping users understand why their VSTest properties are being ignored.

Problem

Users were confused when using VSTest-specific properties with dotnet test while using MTP, because these properties get silently ignored. For example:

• dotnet test --filter something - filter is ignored
• dotnet test --logger trx - logger is ignored

This led to users wondering why their test filtering or logging wasn't working as expected.

Solution

Added comprehensive validation that produces an error when VSTest-specific properties are set with MTP:

Key Changes

1. New opt-out property: TestingPlatformIgnoreVSTestProperties (default: false) allows users to suppress the error when needed.

2. Validation target: Added _ValidateVSTestProperties target that checks for all VSTest properties mentioned in the issue:

   • VSTestSetting, VSTestListTests, VSTestTestCaseFilter, VSTestTestAdapterPath
   • VSTestLogger, VSTestDiag, VSTestResultsDirectory, VSTestCollect
   • VSTestBlame, VSTestBlameCrash, VSTestBlameHang

3. Clear error message: Provides helpful guidance including:

   • Lists which VSTest properties are set
   • Explains they will be ignored with MTP
   • Shows how to suppress the error
   • Links to migration documentation

4. Integration with existing workflow: The validation runs as part of the InvokeTestingPlatform target, ensuring it catches issues early in the build process.

Example Error Message

VSTest-specific properties are set but will be ignored when using Microsoft Testing Platform. 
The following properties are set: VSTestTestCaseFilter; VSTestLogger; VSTestBlame. 
To suppress this error, set TestingPlatformIgnoreVSTestProperties to true. 
For more information about migrating from VSTest to Microsoft Testing Platform, see: https://docs.microsoft.com/en-us/dotnet/core/testing/unit-testing-mstest-intro

Usage Examples

Error case:

dotnet test --filter Category=Unit --logger trx
# Now shows clear error explaining the issue

Opt-out:

dotnet test --filter Category=Unit -p:TestingPlatformIgnoreVSTestProperties=true
# Suppresses the error for migration scenarios

Project file opt-out:

<PropertyGroup>
  <TestingPlatformIgnoreVSTestProperties>true</TestingPlatformIgnoreVSTestProperties>
</PropertyGroup>

Testing

Added comprehensive integration tests covering:

• Error when VSTest properties are set
• No error when opt-out property is enabled
• No error when no VSTest properties are set

Breaking Change Notice

This is a breaking change as it will now show errors for previously "working" (but ignored) VSTest property usage. However, this is intentional to help users identify and fix configuration issues. Users can opt-out during migration using the TestingPlatformIgnoreVSTestProperties property.

Fixes #5697.

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