fix: CreateSubdirectory failing for root directories

[Neo-vortex]

fixes #116087

Fix CreateSubdirectory failing for root directories

Summary

Fixes a bug where DirectoryInfo.CreateSubdirectory(path) incorrectly throws ArgumentException when called on root directory instances (e.g., C:\).

Problem

Path.TrimEndingDirectorySeparator preserves trailing separators for root paths to maintain valid path format. This causes the boundary validation logic to use an incorrect index when checking for directory separators.

Example failure:

var rootDir = new DirectoryInfo(@"C:\");
rootDir.CreateSubdirectory("test"); // Throws ArgumentException

The check at index [3] evaluates the character 't' instead of the separator '\' at index [2], causing validation to fail.

Solution

After calling Path.TrimEndingDirectorySeparator, explicitly check if trimmedCurrentPath still has a trailing separator (root directory case) and remove it manually before performing boundary validation.

if (trimmedCurrentPath.Length > 0 && 
    PathInternal.IsDirectorySeparator(trimmedCurrentPath[trimmedCurrentPath.Length - 1]))
{
    trimmedCurrentPath = trimmedCurrentPath.Slice(0, trimmedCurrentPath.Length - 1);
}

This ensures consistent behavior: trimmedCurrentPath never ends with a separator, making the boundary index check work correctly for all directory types.

Impact

• Fixes: Root directory subdirectory creation (Windows: C:\, D:\, )
• No breaking changes: Non-root directory behavior unchanged
• Low risk: Localized change to validation logic only

Testing

• Existing tests continue to pass (non-root directories)
• Manual testing confirms fix for root directories:

  new DirectoryInfo(@"C:\").CreateSubdirectory("test");     // ✓ Works
  new DirectoryInfo(@"C:\Users").CreateSubdirectory("Docs"); // ✓ Still works

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[kasperk81]

Example failure:

that can be a test case

[Neo-vortex]

I added a test
@kasperk81

[Neo-vortex]

it seems Linux is also effected.
this code fails with the same error without this PR

var dir = new DirectoryInfo("/");
dir.CreateSubdirectory("test");

I added a test for linux as well