Fix Helix tests not running after xUnit v3 migration

[MichaelSimons]

Summary

After the xUnit v2 to v3 migration (#52930), all Helix work items fail with:

No test is available in {assembly}.dll. Make sure that test discoverer & executors are registered and platform & framework version settings are appropriate and try again.

This means zero tests are actually executing on Helix. The ~2k tests visible on the PR Tests tab come from non-Helix sources (FullFramework/AoT legs), down from ~71k tests pre-migration (e.g. PR #53613).

Root Cause

dotnet test {assembly}.dll relies on VSTest discovery to find tests. With xUnit v3, the VSTest adapter (xunit.v3.runner.visualstudio) is not resolvable by the test host on Helix agents — only the SDK's built-in Extensions directory is searched, and the xUnit v3 adapter is not there. The diagnostic log confirms "TestDiscoverers":[] and "ExecutedTests":0.

Fix

Switch from dotnet test to dotnet exec to run xUnit v3 tests as self-hosting executables, matching the approach used by the Arcade Helix SDK's CreateXUnitV3WorkItems task.

Changes

File	Change
SDKCustomCreateXUnitWorkItemsWithTestExclusion.cs	Replace dotnet test with dotnet exec --roll-forward Major. Replace VSTest options (--logger trx, --blame-hang, --filter) with xUnit v3 native options (-xml testResults.xml, -noAutoReporters, -class). Convert env var pass-through from -e to shell export/set.
AssemblyScheduler.cs	Remove trailing . from class names in partition filter string. The dot was needed for VSTest FullyQualifiedName contains-matching; xUnit v3's native -class filter uses exact match.

Before (broken)

dotnet test X.dll --filter "Ns.Class1.|Ns.Class2." --logger trx --blame-hang ...

After (fixed)

dotnet exec --roll-forward Major --runtimeconfig X.runtimeconfig.json --depsfile X.deps.json X.dll -xml testResults.xml -noAutoReporters -nocolor -class Ns.Class1 -class Ns.Class2

Verification

Confirmed via Helix API console logs from PR #53680 (post-xUnit v3) that every work item produces No test is available and ExecutedTests: 0.