Migrate Exec task to TaskEnvironment API

[Copilot]

Context

The Exec task needs to support MSBuild's multithreaded execution model. Tasks that rely on global process state (working directory, environment variables) are not thread-safe and cannot run in parallel.

Changes Made

• Add [MSBuildMultiThreadableTask] attribute and implement IMultiThreadableTask
• Add TaskEnvironment property for thread-safe access to execution environment
• Replace Directory.GetCurrentDirectory() with TaskEnvironment.ProjectDirectory
• Use TaskEnvironment.GetAbsolutePath(WorkingDirectory) for relative path resolution
• Change _workingDirectory field from string to AbsolutePath type for type safety
• Use .OriginalValue for logging (user-facing messages) and .Value for file system operations
• Update all test helpers to set TaskEnvironment = TaskEnvironmentHelper.CreateForTest()
• Add TaskEnvironmentHelper.CreateMultithreadedForTest(projectDirectory) helper method for testing with the multithreaded driver

// Before - uses global process state
_workingDirectory = !string.IsNullOrEmpty(WorkingDirectory)
    ? WorkingDirectory
    : Directory.GetCurrentDirectory();

// After - uses TaskEnvironment with AbsolutePath for thread safety
_workingDirectory = !string.IsNullOrEmpty(WorkingDirectory)
    ? TaskEnvironment.GetAbsolutePath(WorkingDirectory)
    : TaskEnvironment.ProjectDirectory;

Testing

Added 4 comprehensive tests covering both TaskEnvironment drivers and multithreaded scenarios:

• ExecResolvesRelativeWorkingDirectoryWithMultiProcessDriver - Tests relative WorkingDirectory resolution with MultiProcessDriver
• ExecUsesProjectDirectoryAsDefaultWorkingDirectory - Tests default working directory when not specified
• ExecHandlesAbsoluteWorkingDirectory - Tests absolute WorkingDirectory paths
• ExecResolvesRelativeWorkingDirectoryRelativeToProjectDirectory - Uses MultiThreadedTaskEnvironmentDriver to test relative path resolution when process CWD differs from project directory (simulating multithreaded mode)

All 49 Exec tests pass (1 pre-existing Windows-only failure on Linux, 2 correctly skipped)

Notes

• The static Environment.GetEnvironmentVariable("ComSpec") in CommandProcessorPath is unchanged—it's system-level initialization that occurs once before any task execution, and on Unix it returns just "sh" (not a rooted path).
• UNC paths (e.g., \\server\share) work correctly with AbsolutePath since they are fully qualified paths and Path.IsPathFullyQualified() returns true for them.

Original prompt

---

This section details on the original issue you should resolve

<issue_title>Migrate Exec task to TaskEnvironment API</issue_title>
<issue_description></issue_description>

<agent_instructions>Make sure to preserve correctness, performance and that the tests pass</agent_instructions>

Comments on the Issue (you are @copilot in this section)

• Fixes Migrate Exec task to TaskEnvironment API #13170

---

💡 You can make Copilot smarter by setting up custom instructions, customizing its development environment and configuring Model Context Protocol (MCP) servers. Learn more Copilot coding agent tips in the docs.

[JanProvaznik]

improve testing @copilot

[Copilot]

improve testing @copilot

Improved testing in dd8d189 with 4 comprehensive tests covering:

• MultiProcessDriver relative path resolution
• Default working directory behavior (no WorkingDirectory specified)
• Absolute WorkingDirectory handling
• MultiThreadedDriver relative path resolution (simulating multithreaded mode where process CWD differs from project directory)

Also added TaskEnvironmentHelper.CreateMultithreadedForTest(projectDirectory) helper method for testing with the multithreaded driver.

[JanProvaznik]

@copilot

[Copilot]

Pull request overview

Migrates the Exec task toward MSBuild’s TaskEnvironment API so it can participate safely in the multithreaded (/mt) execution model by avoiding reliance on global process state.

Changes:

• Mark Exec as [MSBuildMultiThreadableTask] and implement IMultiThreadableTask with a TaskEnvironment property.
• Resolve/default WorkingDirectory via TaskEnvironment and use AbsolutePath for type-safe filesystem operations and user-facing logging.
• Update/unit-test infrastructure: add a multithreaded TaskEnvironment test helper and update/add Exec tests to supply TaskEnvironment.

Reviewed changes

Copilot reviewed 3 out of 3 changed files in this pull request and generated 5 comments.

File	Description
src/UnitTests.Shared/TaskEnvironmentHelper.cs	Adds CreateMultithreadedForTest helper to construct a TaskEnvironment backed by the multithreaded driver for /mt-style tests.
src/Tasks/Exec.cs	Converts Exec to an IMultiThreadableTask, switches working directory handling to TaskEnvironment, and uses AbsolutePath for safety/clarity.
src/Tasks.UnitTests/Exec_Tests.cs	Ensures tests set TaskEnvironment for Exec instances and adds new tests around working directory resolution behavior.