Enable safe TemplateEngine test parallelization

[Evangelink]

These TemplateEngine test projects currently run serially despite having independent test-class fixtures. This opts them into conservative class-level parallelism so independent classes can overlap without exposing method-scoped shared state to concurrency.

Summary

• Enable MSTestParallelizeScope=ClassLevel in five TemplateEngine utility, search, localization, edge, and discovery test projects.
• Preserve serial execution within classes that share settings helpers, mutable providers, package-manager state, culture-sensitive fixtures, fixed package outputs, or custom hives.
• Avoid broad resource locks and DoNotParallelize because the audited shared state is contained within individual classes.

Validation

Local test and benchmark execution was intentionally deferred so the change can be measured in the coordinated 10-run baseline/changed full-fleet benchmark after integration.

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[Copilot]

Copilot review overview

Review tier: Lite
Findings: None

What changed in this PR

This PR opts five TemplateEngine MSTest.Sdk test projects into conservative class-level parallel execution by overriding the repo default (test/Directory.Build.props sets MSTestParallelizeScope=None). The intent is to allow independent test classes to overlap while keeping methods within each class serialized to avoid class-scoped shared state conflicts.

Changes:

• Set <MSTestParallelizeScope>ClassLevel</MSTestParallelizeScope> in five TemplateEngine test .csproj files.
• Add explanatory comments in each project file describing the intended isolation/serialization boundary (methods serialized within a class; classes may run concurrently).

File	Description
test/​TemplateEngine/​Microsoft.TemplateSearch.TemplateDiscovery.IntegrationTests/​Microsoft.TemplateSearch.TemplateDiscovery.IntegrationTests.csproj	Enables MSTest class-level parallelization for template discovery integration tests while keeping per-class method runs serial.
test/​TemplateEngine/​Microsoft.TemplateSearch.Common.UnitTests/​Microsoft.TemplateSearch.Common.UnitTests.csproj	Enables class-level parallelization for TemplateSearch common unit tests, preserving class-internal serialization for shared fixtures.
test/​TemplateEngine/​Microsoft.TemplateEngine.Utils.UnitTests/​Microsoft.TemplateEngine.Utils.UnitTests.csproj	Enables class-level parallelization for TemplateEngine utils unit tests across TFMs, keeping method runs serial within each class.
test/​TemplateEngine/​Microsoft.TemplateEngine.TemplateLocalizer.Core.UnitTests/​Microsoft.TemplateEngine.TemplateLocalizer.Core.UnitTests.csproj	Enables class-level parallelization for TemplateLocalizer core unit tests with method serialization within each class.
test/​TemplateEngine/​Microsoft.TemplateEngine.Edge.UnitTests/​Microsoft.TemplateEngine.Edge.UnitTests.csproj	Enables class-level parallelization for TemplateEngine edge unit tests, preserving class-internal serialization for shared state.