Modularize project structure by splitting responsibilities into distinct files and namespaces. Add helper methods for authentication and JSON operations.

Helper/Extensions.cs

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+using System.Text;
+using System.Text.Json.Nodes;
+
+namespace FsTool.Helpers
+{
+    public static class Extensions
+    {
+        #region Public Method FsPostAsync
+
+        /// <summary>
+        ///     Sends a POST request to the specified URI with the provided JSON content.
+        /// </summary>
+        /// <param name="httpClient">The instance of <see cref="HttpClient" /> used to send the request.</param>
+        /// <param name="requestUri">The URI to which the request is sent.</param>
+        /// <param name="jsonObject">The JSON object to include in the request body.</param>
+        /// <returns>A task that represents the asynchronous operation. The task result contains the HTTP response.</returns>
+        public static async Task<HttpResponseMessage> FsPostAsync(this HttpClient httpClient, string requestUri, JsonObject jsonObject)
+        {
+            var content = new StringContent(jsonObject.ToString(), Encoding.UTF8, "application/json");
+            content.Headers.ContentType = new("application/json");
+            return await httpClient.PostAsync(requestUri, content);
+        }
+
+        #endregion
+
+        #region Public Method ToList
+
+        /// <summary>
+        ///     Converts the specified JSON node into a list of non-null <see cref="JsonNode" /> elements.
+        /// </summary>
+        /// <param name="node">The <see cref="JsonNode" /> to be converted. If null, an empty list is returned.</param>
+        /// <returns>A list of <see cref="JsonNode" /> instances containing non-null elements.</returns>
+        public static List<JsonNode> ToList(this JsonNode? node)
+        {
+            var array = node?.AsArray() ?? [];
+            return array.Where(x => x != null).Select(x => x!).ToList();
+        }
+
+        #endregion
+    }
+}