Add endpoints for culture detection and improve cookie parsing

Added two endpoints to TestLocalizerController for retrieving culture from Accept-Language and user cookies. Enhanced GetCulture extension to parse and return the actual culture string from cookies. Updated usings for new extension methods.

EnvelopeGenerator.Web/Extensions/WebExtensions.cs

@@ -61,7 +61,18 @@ public static class WebExtensions
 
     #region Cookie
     public static string? GetCulture(this IRequestCookieCollection cookies)
-        => cookies[CookieRequestCultureProvider.DefaultCookieName];
+    {
+        var cookieValue = cookies[CookieRequestCultureProvider.DefaultCookieName];
+
+        if (!string.IsNullOrEmpty(cookieValue))
+        {
+            var culture = CookieRequestCultureProvider.ParseCookieValue(cookieValue)?.Cultures.FirstOrDefault().Value;
+            if (!string.IsNullOrEmpty(culture))
+                return culture;
+        }
+
+        return null;
+    }
 
     public static void SetCulture(this IResponseCookies cookies, string culture)
     {