absolutizing "" should be an error

[JanProvaznik]

Context

aligning with https://learn.microsoft.com/en-us/dotnet/api/system.io.fileinfo?view=net-10.0 to throw on invalid createon

Changes Made

absolutizing "" should throw with a clear exception
fix copy to continue on error

Testing

added unit tests

Notes

[Copilot]

Pull request overview

This PR introduces a new behavior for handling empty and null paths in MSBuild's AbsolutePath class, gated behind ChangeWave 18.4. The change addresses an issue where absolutizing an empty string resulted in BCL path validation errors.

Changes:

• Added a new CreateFromRelative method to centralize path combination logic with changewave-gated validation
• Empty string paths now throw ArgumentException when Wave18.4 is enabled, or return as-is when disabled
• Refactored MultiThreadedTaskEnvironmentDriver and MultiProcessTaskEnvironmentDriver to use the new method
• Added comprehensive test coverage for the new behavior

Reviewed changes

Copilot reviewed 5 out of 5 changed files in this pull request and generated 7 comments.

Show a summary per file

File	Description
src/Framework/PathHelpers/AbsolutePath.cs	Adds CreateFromRelative method with changewave-gated validation for empty/null paths
src/Framework.UnitTests/AbsolutePath_Tests.cs	Adds comprehensive unit tests for CreateFromRelative covering all scenarios with/without Wave18.4
src/Build/BackEnd/TaskExecutionHost/MultiThreadedTaskEnvironmentDriver.cs	Refactors to use new CreateFromRelative method, eliminating code duplication
src/Build/BackEnd/TaskExecutionHost/MultiProcessTaskEnvironmentDriver.cs	Refactors to use new CreateFromRelative method, eliminating code duplication
src/Build.UnitTests/BackEnd/TaskEnvironment_Tests.cs	Updates tests to verify new behavior with Wave18.4 enabled/disabled

[baronfel]

We talked about this today, and the main discussion was around if and how we should throw here. We went to the BCL for inspiration: structures like FileInfo and DirectoryInfo throw on construction if presented with invalid values, and so we should follow that pattern. If users may accept invalid values, then they need to try/catch construction of AbsolutePaths, just like they would with FileInfo/DirectoryInfo. This means a bit more work on Tasks that are enlightening, as you may need to track erroring files/items/etc separately for error reporting, but this is conceptually no different from what would have happened today with the BCL file structures.