[zjdangge]

AcceptExistingOnly() currently accepts directories for FileInfo values. Require File.Exists in the file-specific validation branch, while preserving the existing DirectoryInfo and FileSystemInfo behavior.

Adds regression coverage for scalar and collection arguments/options, plus checks that FileSystemInfo still accepts both files and directories.

Fixes #2723.

Validation on Windows:

• Before the fix, all four new directory-rejection cases fail; the file-existence group has 22 passing and 4 failing cases.
• Debug solution build: 0 warnings and 0 errors.
• All five Debug test assemblies: 1,957 passed, 0 failed, 14 existing skips, including core tests on .NET 10 and .NET Framework 4.7.2.

Tests used English UI culture and disabled xUnit AppDomains. Built assemblies ran from a path without spaces to avoid existing test/script path-handling issues.