sln-add: Don't add solution folders outside the scope of solution directory

[edvilme]

Fixes #48608

Description

When adding a project to a solution file, solution folders are automatically generated with the relative path (relative to the solution file path). When adding projects outside the scope of the solution working directory, solution folders with invalid names are created (e.g., .., :, etc).

This was addressed in #46456 but didn't consider paths in different volumes.

Risk

Low - this doesn't alter existing behavior but rather considers a previously not considered edge case.

Regression

Yes - this worked before 9.0.2xx

Testing

Adding automated tests is difficult as it involves mounting several volumes, but manual tests were performed.

Expected behavior

Current behavior

[Forgind]

Though I generally look favorably upon trying to clean up code, I don't think it's positive in servicing PRs, as it makes it harder to review and can lead to unexpected errors. I found GenerateIntermediateSolutionFoldersForProjectPath particularly confusing. I'm not saying this can't go in, but I don't think it should go into 9.0.3xx as-is.

[Forgind]

So for instance, if I had a path like src\Build\..\..\..\..\test\foo...

[edvilme]

I am not sure it is entirely possible to run into this issue since relativePath is obtained through Path.GetRelativePath, which should return the actual resolved path.
So instead of src\Build\..\..\..\..\test\foo, it should be ..\..\test\foo

[Forgind]

Fair enough; missed that

[Forgind]

I think you could potentially be more lenient with things with .. that end up still in the sln's cone, but 🤷