MSTest V2: in-assembly parallel test execution

Introduces in-assembly parallel test execution for MSTest V2, allowing tests within a single assembly (DLL) to run in parallel. This feature can be enabled globally via .runsettings or through assembly-level annotations. It supports class-level and method-level parallelization, with an option for custom plugins. Opt-out mechanisms using `[DoNotParallelize()]` are provided. The feature composes with existing process-parallelization and distributed test execution in Azure Pipelines.