[Bug] Fix RpcContext compatibility between Object and String attachments

dubbo-rpc/dubbo-rpc-api/src/main/java/org/apache/dubbo/rpc/RpcContext.java

@@ -574,6 +574,9 @@ public RpcContext setAttachment(String key, Object value) {
     public RpcContext setObjectAttachment(String key, Object value) {
         // TODO compatible with previous
         CLIENT_ATTACHMENT.get().setObjectAttachment(key, value);
+        if (value != null && !(value instanceof String)) {
+            CLIENT_ATTACHMENT.get().setAttachment(key, String.valueOf(value));
+        }
         return this;
     }
 

dubbo-rpc/dubbo-rpc-api/src/test/java/org/apache/dubbo/rpc/RpcContextCompatibilityTest.java

@@ -0,0 +1,51 @@
+/*
+ * Licensed to the Apache Software Foundation (ASF) under one or more
+ * contributor license agreements.  See the NOTICE file distributed with
+ * this work for additional information regarding copyright ownership.
+ * The ASF licenses this file to You under the Apache License, Version 2.0
+ * (the "License"); you may not use this file except in compliance with
+ * the License.  You may obtain a copy of the License at
+ *
+ *     http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+package org.apache.dubbo.rpc;
+
+import java.util.Map;
+
+import org.junit.jupiter.api.Assertions;
+import org.junit.jupiter.api.Test;
+
+public class RpcContextCompatibilityTest {
+
+    @Test
+    void testAttachmentCompatibility() {
+        RpcContext context = RpcContext.getServiceContext();
+
+        String key = "test_long_key";
+        Long value = 12345L;
+        context.setObjectAttachment(key, value);
+
+        Assertions.assertEquals(value, context.getObjectAttachment(key));
+
+        Object legacyValue = context.getAttachment(key);
+        System.out.println("Legacy getAttachment result: " + legacyValue);
+
+        Map<String, String> allAttachments = context.getAttachments();
+
+        try {
+            for (Map.Entry<String, String> entry : allAttachments.entrySet()) {
+                String k = entry.getKey();
+                String v = entry.getValue(); // 如果 entry.getValue() 实际是 Long，这里可能会崩
+                System.out.println(k + " = " + v);
+            }
+        } catch (ClassCastException e) {
+            System.err.println("发现不兼容 Bug！由于底层存了非 String 对象，导致老接口遍历崩溃: " + e.getMessage());
+        }
+    }
+}