Add a custom toString to DynamicMap (#126562)

This change prevents a delegate map from giving an inconsistent 
toString or an empty toString.

Closes #70262

Author: jdconrad

docs/changelog/126562.yaml

@@ -0,0 +1,6 @@
+pr: 126562
+summary: Add a custom `toString` to `DynamicMap`
+area: Infra/Scripting
+type: bug
+issues:
+ - 70262

server/src/main/java/org/elasticsearch/script/DynamicMap.java

@@ -98,6 +98,17 @@ public Set<Entry<String, Object>> entrySet() {
 
     @Override
     public String toString() {
-        return delegate.toString();
+        StringBuilder sb = new StringBuilder("{");
+        int count = 0;
+        for (Map.Entry<String, Object> entry : entrySet()) {
+            sb.append(entry.getKey());
+            sb.append("=");
+            sb.append(entry.getValue());
+            if (++count < size()) {
+                sb.append(", ");
+            }
+        }
+        sb.append("}");
+        return sb.toString();
     }
 }

server/src/test/java/org/elasticsearch/script/ScriptTests.java

@@ -29,6 +29,8 @@
 import java.util.HashMap;
 import java.util.Map;
 
+import static org.hamcrest.Matchers.both;
+import static org.hamcrest.Matchers.containsString;
 import static org.hamcrest.Matchers.equalTo;
 
 public class ScriptTests extends ESTestCase {
@@ -192,7 +194,6 @@ public void testDynamicMapToString() {
         map.put("long", 1L);
         map.put("string", "value");
         DynamicMap dm = new DynamicMap(map, Collections.emptyMap());
-        assertTrue(dm.toString().contains("string=value"));
-        assertTrue(dm.toString().contains("long=1"));
+        assertThat(dm.toString(), both(containsString("long=1")).and(containsString("string=value")));
     }
 }