[Multithreaded] Migrate RunReadyToRunCompiler

[AlesProkop]

Related to dotnet/msbuild#13073

Context

RunReadyToRunCompiler used process-global state when resolving tool, input, and output paths. The task needs per-project path and process environment handling before it can execute safely as a multithreaded MSBuild task.

Changes Made

• Added MSBuildMultiThreadableTask and IMultiThreadableTask support with a TaskEnvironment.Fallback default.
• Absolutized filesystem and executable paths through TaskEnvironment while preserving original values in diagnostics and compiler arguments.
• Added a .NET Framework ToolTask.GetProcessStartInfo compatibility override to apply the task-local working directory and environment.
• Added focused multithreading tests for independent project-directory resolution and empty tool-path diagnostics.

Testing

• Built Microsoft.NET.Build.Tasks.csproj for net11.0 successfully.
• Built Microsoft.NET.Build.Tasks.csproj for net472 successfully.
• Added and ran focused tests in GivenARunReadyToRunCompilerMultiThreading:
  • RelativePathsResolveIndependentlyForEachTask verifies relative paths resolve against each task instance's project directory.
  • EmptyToolPathUsesExistingValidationDiagnostic verifies empty paths retain the existing validation failure behavior.
  • Result: 2 passed, 0 failed.

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