fix(core): make IProtoResponseImpl.toString() idempotent across repeated calls

tarantool-core/src/main/java/io/tarantool/core/protocol/IProtoResponseImpl.java

@@ -21,7 +21,6 @@
 
 public class IProtoResponseImpl implements IProtoMessage {
 
-  private StringBuilder stringBuilder;
   protected MapValue header;
   protected MapValue body;
   protected Map<Integer, Value> headerMap;
@@ -51,19 +50,13 @@ protected static Map<Integer, Value> convertMap(MapValue map) {
 
   @Override
   public String toString() {
-    if (this.stringBuilder == null) {
-      this.stringBuilder = new StringBuilder();
-    }
-
-    this.stringBuilder.append("IProtoResponseImpl(header = ").append(getHeader().toJson());
+    String result = "IProtoResponseImpl(header = " + getHeader().toJson();
 
     if (getBody() != null) {
-      this.stringBuilder.append(", body = ").append(getBody().toJson());
+      result += ", body = " + getBody().toJson();
     }
 
-    this.stringBuilder.append(")");
-
-    return this.stringBuilder.toString();
+    return result + ")";
   }
 
   @Override

tarantool-core/src/test/java/io/tarantool/core/unit/protocol/IProtoResponseImplTest.java

@@ -0,0 +1,31 @@
+/*
+ * Copyright (c) 2026 VK DIGITAL TECHNOLOGIES LIMITED LIABILITY COMPANY
+ * All Rights Reserved.
+ */
+
+package io.tarantool.core.unit.protocol;
+
+import static org.junit.jupiter.api.Assertions.assertEquals;
+import static org.junit.jupiter.api.Assertions.assertTrue;
+import org.junit.jupiter.api.Test;
+import org.msgpack.value.ValueFactory;
+
+import static io.tarantool.core.protocol.requests.IProtoConstant.MP_IPROTO_REQUEST_TYPE;
+import io.tarantool.core.protocol.IProtoResponseImpl;
+
+public class IProtoResponseImplTest {
+
+  @Test
+  public void testToStringDoesNotDuplicateOnRepeatedCalls() {
+    IProtoResponseImpl response =
+        new IProtoResponseImpl(
+            ValueFactory.newMap(MP_IPROTO_REQUEST_TYPE, ValueFactory.newInteger(0)),
+            ValueFactory.emptyMap());
+
+    String first = response.toString();
+    String second = response.toString();
+
+    assertTrue(first.startsWith("IProtoResponseImpl(header = "));
+    assertEquals(first, second);
+  }
+}