Track downstream-repo migration for Helix Sdk XUnitV3Project -> MTPProject rename (breaking)

[Evangelink]

Spun off from review feedback on #16986 by @Youssef1313 (#16986 (comment) and #16986 (comment)).

Context

PR #16986 renamed the Helix Sdk CreateXUnitV3WorkItems task / XUnitV3Project item to CreateMTPWorkItems / MTPProject and unconditionally produces an MTP command line that:

1. Requires the test project to reference Microsoft.Testing.Extensions.TrxReport (previously: xUnit XML reporter).
2. Emits --report-trx --report-trx-filename testResults.trx (previously: --report-xunit-xml --report-xunit-xml-filename testResults.xml or legacy -xml testResults.xml -noAutoReporters depending on UseMicrosoftTestingPlatformRunner).

This is intentional and the PR is flagged as breaking. The concern raised in review is that downstream repos consuming the Helix Sdk may still be on:

• xUnit v3 without MTP enabled (UseMicrosoftTestingPlatformRunner previously gated this), or
• xUnit v3 with MTP enabled but producing xUnit-format reports instead of TRX.

Both groups will see a hard break the first time they pick up the new Helix Sdk.

Action items

• Enumerate downstream repos using <XUnitV3Project> (search across dotnet/* and known consumers).
• For each repo, file a migration PR or open an issue documenting the rename and the new requirement to reference Microsoft.Testing.Extensions.TrxReport.
• Update the .NET 11 / Arcade release notes / breaking change announcement.
• (Optional) Consider a one-release deprecation window where the old CreateXUnitV3WorkItems task is kept as a thin shim forwarding to CreateMTPWorkItems with a warning.

cc @Youssef1313 @ilyas1974