Fix DirectoryInfo.CreateSubdirectory for root directories

[Copilot]

DirectoryInfo.CreateSubdirectory was incorrectly throwing ArgumentException when called on root directories (like "/" on Unix or "C:\" on Windows), preventing users from creating subdirectories under the filesystem root.

Problem

The validation logic in CreateSubdirectory expected that the character immediately following the current directory path would be a directory separator when validating that the new path is a proper subdirectory. However, for root directories, Path.Combine produces paths where this character is actually the first letter of the subdirectory name, not a separator.

For example:

• Current path: "/"
• Subdirectory: "test"
• Combined path: "/test"
• Character at position 1: 't' (not a directory separator)

This caused the validation to fail and throw: "The directory specified, 'test', is not a subdirectory of '/'"

Solution

Added a special case in the validation logic to allow subdirectory creation when the current path is a root directory by checking PathInternal.IsRoot(trimmedCurrentPath). This preserves all existing security validations while enabling the legitimate use case of creating subdirectories under root directories.

Testing

• ✅ All existing CreateSubdirectory tests continue to pass (15/15)
• ✅ All DirectoryInfo tests pass (1575 passed, 17 skipped)
• ✅ Added new test case that verifies root directory subdirectory creation
• ✅ Security validations remain intact (parent traversal, rooted paths still properly rejected)
• ✅ Edge cases work correctly (trailing separators, nested paths, etc.)

The fix is minimal and surgical, changing only the specific validation condition while preserving all existing behavior.

Fixes #116087.

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