refactor(json): drop unused isEmpty() and consolidate redundant fuzz tests

- Delete JSONObject.isEmpty() (no callers in main or test code)
- Promote size() and keySet() to canonical position with proper Javadoc;
  both still required by HttpMethed test util and stay drop-in compatible
- Fuzz test consolidation (165 -> 132 methods):
  * Merge §50 boolean->numeric coercion (5 single-getter tests -> 1)
  * Drop §52 boxed null-string redundancies (covered by §17/§18)
  * Trim §28 toJSONString unwrap suite (9 -> 4)
  * Trim equals tests (Array 8 -> 4, Object 10 -> 4); cover reflexivity,
    nested, empty and built-vs-parsed inside the SameContent canonical case
  * Replace .size()/.isEmpty() asserts with containsKey()/toJSONString()
    where the original assertion was redundant

Author: halibobo1205

common/src/main/java/org/tron/json/JSONObject.java

@@ -5,6 +5,7 @@
 import com.fasterxml.jackson.databind.node.ArrayNode;
 import com.fasterxml.jackson.databind.node.ObjectNode;
 import java.math.BigDecimal;
+import java.util.Iterator;
 import java.util.LinkedHashSet;
 import java.util.List;
 import java.util.Set;
@@ -50,6 +51,24 @@ public boolean containsKey(String key) {
     return node.has(key);
   }
 
+  /** Mirrors {@code Map.size()} from fastjson's JSONObject. */
+  public int size() {
+    return node.size();
+  }
+
+  /**
+   * Mirrors {@code Map.keySet()} from fastjson's JSONObject.
+   * Returns a {@link LinkedHashSet} preserving insertion / parse order.
+   */
+  public Set<String> keySet() {
+    Set<String> keys = new LinkedHashSet<>(node.size());
+    Iterator<String> names = node.fieldNames();
+    while (names.hasNext()) {
+      keys.add(names.next());
+    }
+    return keys;
+  }
+
   /**
    * Returns the string representation of the value for the given key.
    * For object/array nodes, returns the JSON text (matching Fastjson behaviour).
@@ -433,20 +452,6 @@ public Object remove(String key) {
     return convertNode(removed);
   }
 
-  public boolean isEmpty() {
-    return node.isEmpty();
-  }
-
-  public int size() {
-    return node.size();
-  }
-
-  public Set<String> keySet() {
-    Set<String> keys = new LinkedHashSet<>();
-    node.fieldNames().forEachRemaining(keys::add);
-    return keys;
-  }
-
   /** Returns the underlying Jackson {@link ObjectNode}. */
   @JsonValue
   public ObjectNode unwrap() {