Migrate tests to xUnit 4 and MTP retry

[Sebastien Lebreton (sailro)]

Summary

Migrate the analyzer test project to xUnit.net v3 4.0.0 and Microsoft.Testing.Platform (MTP) v2, and replace the legacy VSTest CI path with native MTP execution and retry support.

This change:

• updates xunit.v3 and xunit.runner.visualstudio to 4.0.0
• adds Microsoft.Testing.Extensions.Retry 2.3.3
• configures the test project with UseMicrosoftTestingPlatformRunner
• runs the built test assembly directly instead of invoking VSTest-mode dotnet test
• enables up to three retries for failed tests
• preserves the existing distinction between full main-branch runs and feature/PR runs that exclude ConsistencyTests
• updates the README, CI documentation, and Copilot instructions with the new runner commands
• opts CI out of Microsoft Testing Platform telemetry

This also unblocks the corresponding xUnit 4 migrations in VSTU and VSTUC, which consume this repository through the external\VSTUAnalyzers submodule.

CI behavior

The main branch runs the complete suite:

dotnet ./src/Microsoft.Unity.Analyzers.Tests/bin/Debug/net10.0/Microsoft.Unity.Analyzers.Tests.dll --retry-failed-tests 3

Feature and pull-request branches exclude the documentation-dependent consistency tests:

dotnet ./src/Microsoft.Unity.Analyzers.Tests/bin/Debug/net10.0/Microsoft.Unity.Analyzers.Tests.dll --retry-failed-tests 3 --filter-not-class Microsoft.Unity.Analyzers.Tests.ConsistencyTests

The generated xUnit 4 runner help was used to confirm the retry and exact-class filter syntax rather than relying on the previous VSTest filter format.

Validation

Validation was performed with .NET SDK 10.0.400:

• built the complete solution successfully with no warnings or errors
• ran the feature/PR command successfully: 472 tests passed
• ran the full main-branch command successfully: 476 tests passed
• confirmed through generated --help output that --retry-failed-tests and --filter-not-class are available
• confirmed through generated --info output that Microsoft.Testing.Platform 2.3.3, the xUnit 4.0.0 provider, and Retry 2.3.3 are active