Fix error CS8347, usage of usingNamespaces exceeds its lifetime

Caught by .NET 7.0 compiler.

Author: andyleejordan

src/PowerShellEditorServices/Utility/FormatUtils.cs

@@ -23,6 +23,8 @@ internal static class FormatUtils
 
         private const string Static = "static ";
 
+        private static HashSet<string>? usingNamespaces;
+
         /// <summary>
         /// Space, new line, carriage return and tab.
         /// </summary>
@@ -164,9 +166,9 @@ private static string GetMethodDocumentation(ReadOnlySpan<char> toolTip, out Mar
         {
             kind = MarkupKind.Markdown;
             StringBuilder text = new();
-            HashSet<string>? usingNamespaces = null;
             while (true)
             {
+                usingNamespaces = null;
                 toolTip = toolTip.TrimStart(s_whiteSpace.Span);
                 toolTip = ProcessMethod(toolTip, text, ref usingNamespaces);
                 if (toolTip.IsEmpty)