Move the .NET test framework/platform migration skills (migrate-mstest-v1v2-to-v3, migrate-mstest-v3-to-v4, migrate-vstest-to-mtp, migrate-xunit-to-mstest, migrate-xunit-to-xunit-v3) and the test-migration orchestrator agent out of dotnet-test into a new dedicated dotnet-test-migration plugin, along with their evals. Update all marketplace manifests, READMEs, CODEOWNERS, and .vally.yaml accordingly.
Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>