Add MSBUILDFORCEMULTITHREADED env var to force multi-threaded mode

[Copilot]

Fixes #13606

Context

There was no environment-variable equivalent of the -multiThreaded / -mt switch, making it awkward to opt in to multi-threaded mode in scenarios where the command line cannot be modified (CI, IDE, wrapper scripts). Other switches (e.g. MSBUILDDISABLENODEREUSE for -nodeReuse) already follow this pattern.

Changes Made

• src/Framework/Traits.cs — added Traits.ForceMultiThreaded, reading MSBUILDFORCEMULTITHREADED == "1", alongside the existing ForceAllTasksOutOfProcToTaskHost.
• src/MSBuild/XMake.cs — IsMultiThreadedEnabled short-circuits to true when Traits.Instance.ForceMultiThreaded is set; otherwise falls back to the existing parameterized switch. The method was promoted from private to internal for direct unit-test coverage (matching the pattern of neighboring ProcessMaxCPUCountSwitch).
• documentation/wiki/MSBuild-Environment-Variables.md — documented the new variable next to the other MSBUILDFORCE* entries.
• src/MSBuild.UnitTests/XMake_Tests.cs — added three unit tests for the parsing/trait-resolution logic:
  • env var = "1" → multi-threaded enabled even without -mt.
  • env var unset → not enabled.
  • env var = non-"1" value (e.g. "true") → not enabled (only literal "1" is honored, matching neighboring MSBUILDFORCE* flags).
• src/MSBuild.UnitTests/MSBuildMultithreaded_Tests.cs — added an end-to-end integration test (MSBuildForceMultiThreadedEnvironmentVariablePropagatesToBuildParameters) that launches a real MSBuild process with MSBUILDFORCEMULTITHREADED=1 and no /mt switch, then runs the existing EnvironmentIsolationTestTask with IsMultithreadedMode='true'. The task observes whether TaskEnvironment is isolated from the global environment, which is only true when BuildParameters.MultiThreaded == true. This proves the env var flows end-to-end: Environment → Traits.ForceMultiThreaded → IsMultiThreadedEnabled → local multiThreaded → parameters.MultiThreaded.

internal static bool IsMultiThreadedEnabled(CommandLineSwitches commandLineSwitches)
{
    if (Traits.Instance.ForceMultiThreaded)
    {
        return true;
    }

    return commandLineSwitches.IsParameterizedSwitchSet(CommandLineSwitches.ParameterizedSwitch.MultiThreaded);
}

Testing

• dotnet build src/MSBuild/MSBuild.csproj and src/MSBuild.UnitTests/Microsoft.Build.CommandLine.UnitTests.csproj — 0 warnings, 0 errors.
• All 4 new tests pass when run via the Microsoft.Build.CommandLine.UnitTests runner with --filter-method "*MSBuildForceMultiThreaded*" (4 succeeded, 0 failed).
• The new integration test's build log explicitly shows MultiThreaded Mode - TaskEnvironment is isolated from global environment (PASS), confirming the env var propagates to BuildParameters.MultiThreaded.

Notes

• Env var is opt-in and only honored when set to exactly "1", matching the convention of neighboring MSBUILDFORCE* flags.
• Command-line switch behavior is unchanged when the env var is unset.
• The longer name MSBUILDFORCEMULTITHREADED was chosen over the initial MSBUILDFORCEMT for clarity, per reviewer feedback.

[Copilot]

Pull request overview

Adds an environment-variable escape hatch to force MSBuild’s multi-threaded (in-proc node) mode, mirroring existing patterns where env vars can drive CLI behavior when command lines can’t be modified.

Changes:

• Introduces Traits.ForceMultiThreaded backed by MSBUILDFORCEMT=1.
• Updates XMake.IsMultiThreadedEnabled to short-circuit to multi-threaded mode when the trait is set.
• Documents the new environment variable in the environment-variable reference.

Reviewed changes

Copilot reviewed 3 out of 3 changed files in this pull request and generated 2 comments.

File	Description
src/MSBuild/XMake.cs	Honors Traits.Instance.ForceMultiThreaded to force-enable multi-threaded mode.
src/Framework/Traits.cs	Adds a new trait that reads MSBUILDFORCEMT from the environment.
documentation/wiki/MSBuild-Environment-Variables.md	Documents MSBUILDFORCEMT=1 alongside other MSBUILDFORCE* variables.

[AR-May]

Consider adding some tests for the parameters parsing logic.

[Copilot]

Consider adding some tests for the parameters parsing logic.

Added three unit tests for the parsing/trait-resolution logic in IsMultiThreadedEnabled covering env var = "1", unset, and non-"1" values. (a92ec5c)