fix(extension): 重命名StringExtensions类为StringExtension

将StringExtensions类统一重命名为StringExtension，保持与其他扩展类命名规范一致

Author: AlianBlank

Runtime/Extension/Extension/StringExtensions.cs

@@ -14,7 +14,7 @@ namespace GameFrameX.Runtime
     /// Provides extension methods for string manipulation including comparison, conversion, and validation.
     /// </remarks>
     [UnityEngine.Scripting.Preserve]
-    public static class StringExtensions
+    public static class StringExtension
     {
         /// <summary>
         /// 快速比较两个字符串内容是否一致

Runtime/GameFrameXCroppingHelper.cs

@@ -153,7 +153,7 @@ private void Start()
             _ = typeof(GameFrameX.Runtime.ZipHelper);
             _ = typeof(ObjectExtension);
             _ = typeof(SpanExtension);
-            _ = typeof(StringExtensions);
+            _ = typeof(StringExtension);
             _ = typeof(GameFrameX.Runtime.BidirectionalDictionary<,>);
             _ = typeof(GameFrameX.Runtime.CollectionExtensions);
             _ = typeof(GameFrameX.Runtime.DateTimeExtensions);