Fix handling of root on Unix in directory error handling (#20049)

Author: stephentoub

src/System.IO.FileSystem/src/System/IO/PathHelpers.Unix.cs

@@ -27,5 +27,10 @@ internal static void CheckSearchPattern(string searchPattern)
                 }
             }
         }
+
+        internal static string TrimEndingDirectorySeparator(string path) =>
+            path.Length > 1 && PathInternal.IsDirectorySeparator(path[path.Length - 1]) ? // exclude root "/"
+                path.Substring(0, path.Length - 1) :
+                path;
     }
 }

src/System.IO.FileSystem/src/System/IO/PathHelpers.Windows.cs

@@ -118,5 +118,10 @@ internal static string GetFullSearchString(string fullPath, string searchPattern
 
             return tempStr;
         }
+
+        internal static string TrimEndingDirectorySeparator(string path) =>
+            EndsInDirectorySeparator(path) ?
+                path.Substring(0, path.Length - 1) :
+                path;
     }
 }

src/System.IO.FileSystem/src/System/IO/PathHelpers.cs

@@ -38,12 +38,5 @@ internal static bool EndsInDirectorySeparator(String path)
         {
             return path.Length > 0 && PathInternal.IsDirectorySeparator(path[path.Length - 1]);
         }
-
-        internal static string TrimEndingDirectorySeparator(string path)
-        {
-            return EndsInDirectorySeparator(path) ?
-                path.Substring(0, path.Length - 1) :
-                path;
-        }
     }
 }