WinMDExp Migration

[AlesProkop]

Fixes #13635

Context

Migrates WinMDExp to the multithreaded task model so its path handling, environment access, and winmdexp.exe process
launch go through TaskEnvironment.

Changes Made

• Added multithreaded task support with [MSBuildMultiThreadableTask] and IMultiThreadableTask.
• Absolutized task input/tool paths through TaskEnvironment.GetAbsolutePath() before filesystem probes and tool
  execution.
• Routed SDK lookup environment access through TaskEnvironment.GetEnvironmentVariable().
• Routed winmdexp.exe process creation through TaskEnvironment.GetProcessStartInfo().
• Preserved output property values so absolutized internal paths do not leak into task outputs.

Testing

• Built successfully with no new warnings.
• Ran existing relevant tests.
• Audited output/error path behavior for Sin 1 and Sin 2 concerns.

[Copilot]

Pull request overview

This draft PR starts migrating the built-in WinMDExp task toward MSBuild’s multithreaded execution model. In the codebase, WinMDExp is a Windows-only ToolTask wrapper around winmdexp.exe, so the intended fit is to move path/process interactions onto TaskEnvironment-safe patterns.

Changes:

• Marks the .NET Framework WinMDExp task with [MSBuildMultiThreadableTask].
• Updates GenerateFullPathToTool() to use a TaskEnvironment-aware existence probe during SDK tool discovery.
• Updates SkipTaskExecution() to absolutize the input/output paths used for timestamp comparison.