Enable EnumerateDirectoryWithTrailingSpacePeriod test on all platforms

- Removed [PlatformSpecific(TestPlatforms.Windows)] attribute
- Updated FileSystemEnumerableFactory to use conditional compilation for PathInternal.IsExtended
- On Unix, always trim trailing spaces/periods (no extended syntax concept)
- On Windows, skip trimming only for paths with \\?\ or \\.\  prefix

Co-authored-by: jkotas <6668460+jkotas@users.noreply.github.com>

Author: Copilot

src/libraries/System.Private.CoreLib/src/System/IO/Enumeration/FileSystemEnumerableFactory.cs

@@ -51,14 +51,20 @@ internal static bool NormalizeInputs(ref string directory, ref string expression
             //   "."             → "."             (relative path reference)
             //   ".."            → ".."            (parent directory reference)
             //   " "             → " "             (only spaces - would result in empty)
-            //   "\\?\C:\test."  → "\\?\C:\test." (extended path syntax - no normalization)
+            //   "\\?\C:\test."  → "\\?\C:\test." (extended path syntax on Windows - no normalization)
             //
             // Algorithm: Trim trailing spaces/periods, but only if:
             // 1. Result is non-empty
-            // 2. Path does not use extended syntax (\\?\ or \\.\)
+            // 2. On Windows: Path does not use extended syntax (\\?\ or \\.\)
 
-            // Don't trim paths using extended syntax (\\?\ or \\.\) as they explicitly disable normalization
-            if (!PathInternal.IsExtended(directory.AsSpan()))
+            // Don't trim paths using extended syntax on Windows (\\?\ or \\.\) as they explicitly disable normalization
+            // On Unix, there's no extended syntax concept, so we always trim (unless result would be empty)
+            bool shouldTrim = true;
+#if WINDOWS
+            shouldTrim = !PathInternal.IsExtended(directory.AsSpan());
+#endif
+
+            if (shouldTrim)
             {
                 string trimmed = directory.TrimEnd(' ', '.');
 

src/libraries/System.Runtime/tests/System.IO.FileSystem.Tests/Directory/GetFiles.cs

@@ -223,7 +223,6 @@ public void WindowsEnumerateFilesWithTrailingSpacePeriod(string fileName)
 
         [Theory]
         [MemberData(nameof(TestData.WindowsTrailingProblematicFileNames), MemberType = typeof(TestData))]
-        [PlatformSpecific(TestPlatforms.Windows)]
         public void EnumerateDirectoryWithTrailingSpacePeriod(string dirName)
         {
             DirectoryInfo parentDir = Directory.CreateDirectory(GetTestFilePath());