Limit dotnet-new integration test parallelism

[Copilot]

The dotnet-new Linux Helix work item timed out while running template integration tests. Class-level parallelism allowed too many resource-intensive dotnet and MSBuild child processes to compete concurrently.

• Bound resource usage
  • Limit MSTest execution to two workers.
  • Preserve class-level parallelism while reducing Helix resource contention.

<MSTestParallelizeScope>ClassLevel</MSTestParallelizeScope>
<MSTestParallelizeWorkers>2</MSTestParallelizeWorkers>

• Fixes [AI discovered CI] dotnet-new integration test work item times out during DotnetCSharpClassTemplatesTest #56007

[azure-pipelines]

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