Add ReCClientHelpers for query and JSON request helpers

Introduced internal static class ReCClientHelpers with methods to build query strings from key/value pairs and to create JsonContent payloads for HTTP requests. Includes conditional compilation for .NET Framework compatibility and XML documentation for both methods. Added necessary using directives.

src/ReC.Client/ReCClientHelpers.cs

@@ -0,0 +1,49 @@
+using System;
+using System.Globalization;
+using System.Linq;
+using System.Net.Http.Json;
+
+#if NETFRAMEWORK
+using System.Net.Http;
+#endif
+
+namespace ReC.Client
+{
+    /// <summary>
+    /// Provides shared helpers for composing requests.
+    /// </summary>
+    internal static class ReCClientHelpers
+    {
+#if NETFRAMEWORK
+        /// <summary>
+        /// Builds a query string from the provided key/value pairs, skipping null values.
+        /// </summary>
+        /// <param name="parameters">The key/value pairs to include in the query string.</param>
+        /// <returns>A query string beginning with '?', or an empty string if no values are provided.</returns>
+        public static string BuildQuery(params (string Key, object Value)[] parameters)
+#else
+        /// <summary>
+        /// Builds a query string from the provided key/value pairs, skipping null values.
+        /// </summary>
+        /// <param name="parameters">The key/value pairs to include in the query string.</param>
+        /// <returns>A query string beginning with '?', or an empty string if no values are provided.</returns>
+        public static string BuildQuery(params (string Key, object? Value)[] parameters)
+#endif
+        {
+            var parts = parameters
+                .Where(p => p.Value != null)
+                .Select(p => $"{Uri.EscapeDataString(p.Key)}={Uri.EscapeDataString(Convert.ToString(p.Value, CultureInfo.InvariantCulture) ?? string.Empty)}");
+
+            var query = string.Join("&", parts);
+            return string.IsNullOrWhiteSpace(query) ? string.Empty : $"?{query}";
+        }
+
+        /// <summary>
+        /// Creates a JSON content payload from the provided object.
+        /// </summary>
+        /// <typeparam name="T">The type of the payload.</typeparam>
+        /// <param name="payload">The payload to serialize.</param>
+        /// <returns>A <see cref="JsonContent"/> instance ready for HTTP requests.</returns>
+        public static JsonContent ToJsonContent<T>(T payload) => JsonContent.Create(payload);
+    }
+}