Refactor IsLocalhost to static method

Changed `IsLocalhost` from an instance method to a static method. Updated the parameter to use `host` directly instead of `parsedAddress.Host`. Modified the check for the `.localhost` TLD to ensure the host length is greater than 10, optimizing the condition to avoid unnecessary checks for shorter hostnames.

Fixes #61155

Author: ladeak

src/Servers/Kestrel/Core/src/Internal/AddressBinder.cs

@@ -142,18 +142,16 @@ internal static ListenOptions ParseAddress(string address, out bool https)
 
         return options;
 
-        bool IsLocalhost(string host, out string? prefix)
+        static bool IsLocalhost(string host, out string? prefix)
         {
             // Check for "localhost"
-            if (string.Equals(parsedAddress.Host, "localhost", StringComparison.OrdinalIgnoreCase))
+            if (string.Equals(host, "localhost", StringComparison.OrdinalIgnoreCase))
             {
                 prefix = null;
                 return true;
             }
 
-            // Check for use of .localhost TLD (Top Level Domain)
-            if (host.EndsWith(".localhost", StringComparison.OrdinalIgnoreCase)
-                && !string.Equals(parsedAddress.Host, ".localhost", StringComparison.OrdinalIgnoreCase))
+            if (host.Length > 10 && host.EndsWith(".localhost", StringComparison.OrdinalIgnoreCase))
             {
                 // Take all but the .localhost TLD as the prefix
                 prefix = host[..^10]; // 10 is the length of ".localhost"