Only call ToSpan() once

sdk/src/Core/Amazon.Util/AWSSDKUtils.cs

@@ -200,21 +200,22 @@ public static string GetExtension(string path)
 
 #if NET8_0_OR_GREATER
             // LastIndexOf and LastIndexOfAny is vectorized on .NET8+ and is
-            // signifigantly faster for cases where 'path' does not end with a short file
+            // significantly faster for cases where 'path' does not end with a short file
             // extension, such as GUIDs
-            int extensionIndex = path.AsSpan().LastIndexOf('.');
+            ReadOnlySpan<char> pathSpan = path.AsSpan();
+            int extensionIndex = pathSpan.LastIndexOf('.');
             if (extensionIndex == -1)
             {
                 return string.Empty;
             }
 
-            int directoryIndex = path.AsSpan().LastIndexOfAny('/', '\\', ':');
+            int directoryIndex = pathSpan.LastIndexOfAny('/', '\\', ':');
 
             // extension separator is found and exists before path separator or path separator doesn't exist
             // AND it's not the last one in the string
-            if (directoryIndex < extensionIndex && extensionIndex < path.Length - 1)
+            if (directoryIndex < extensionIndex && extensionIndex < pathSpan.Length - 1)
             {
-                return path.Substring(extensionIndex);
+                return pathSpan.Slice(extensionIndex).ToString();
             }
 
             return string.Empty;