Expose public API for TaskEnvironment construction

[AR-May]

Context

With the discovery of the "direct instantiation of the task" use case, tasks must explicitly set a default task environment in order to work correctly. Task authors may encounter the same issue, without having benefit of having framework internals visible to them and thus without ability to create one without re-implementing the driver themselves. So, we need to provide a public API for TaskEnvironment construction to enable setting the default task environment. Would be helpful for sdk as well.

Changes Made

Adds two public API members to TaskEnvironment so that task authors using IMultiThreadableTask can obtain or create a TaskEnvironment without depending on internal types:

• TaskEnvironment.Default: static singleton that delegates to the process-level environment (multi-process mode).
• TaskEnvironment.CreateMultithreaded(projectDirectory, environmentVariables?): factory method that creates an isolated environment for multithreaded execution mode.

Testing

Unit tests

Note

Depends on #13380

[Copilot]

Pull request overview

This PR aims to make Microsoft.Build.Framework.TaskEnvironment directly instantiable by task authors by adding a public parameterless constructor that selects a default environment driver.

Changes:

• Add a public TaskEnvironment() constructor intended to default to the multi-process driver.

[AR-May]

/azp run

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