fix(json): remove invalid plus signs before parsing config

v2rayN/ServiceLib/Common/JsonUtils.cs

@@ -93,7 +93,8 @@ public class JsonUtils
             {
                 return null;
             }
-            return JsonNode.Parse(strJson, nodeOptions: null, _defaultDocumentOptions);
+
+            return JsonNode.Parse(NormalizeBrokenJson(strJson), nodeOptions: null, _defaultDocumentOptions);
         }
         catch
         {
@@ -167,4 +168,74 @@ public class JsonUtils
     {
         return JsonSerializer.SerializeToNode(obj, options);
     }
+
+    /// <summary>
+    /// Normalizes a broken JSON-like string by removing plus signs (+)
+    /// that are located outside quoted string values.
+    /// </summary>
+    /// <param name="json">
+    /// The JSON-like input string that may contain invalid plus signs.
+    /// </param>
+    /// <returns>
+    /// A normalized JSON string that can be parsed by <see cref="JsonNode.Parse(string?, JsonNodeOptions?, JsonDocumentOptions)"/>.
+    /// </returns>
+    /// <remarks>
+    /// This method is designed for cases where the input looks similar to JSON
+    /// but contains invalid plus signs, such as:
+    ///
+    /// <code>
+    /// {"scMaxEachPostBytes":+1000000,+"scMaxConcurrentPosts":+100,+"xPaddingBytes":+"100-1000"}
+    /// </code>
+    ///
+    /// The method removes only plus signs that are outside string values.
+    ///
+    /// It does not remove plus signs inside quoted strings:
+    ///
+    /// <code>
+    /// {"phone":"+49123456789"}
+    /// </code>
+    ///
+    /// This behavior prevents accidental modification of valid string data.
+    /// </remarks>
+    private static string NormalizeBrokenJson(string json)
+    {
+        var result = new StringBuilder(json.Length);
+
+        bool insideString = false;
+        bool escaped = false;
+
+        foreach (char ch in json)
+        {
+            if (escaped)
+            {
+                result.Append(ch);
+                escaped = false;
+                continue;
+            }
+
+            if (ch == '\\' && insideString)
+            {
+                result.Append(ch);
+                escaped = true;
+                continue;
+            }
+
+            if (ch == '"')
+            {
+                insideString = !insideString;
+                result.Append(ch);
+                continue;
+            }
+
+            // Remove plus signs only when they are outside string values.
+            if (!insideString && ch == '+')
+            {
+                continue;
+            }
+
+            result.Append(ch);
+        }
+
+        return result.ToString();
+    }
 }