fix: 使用终态答案校正流式输出

- DONE 事件携带后端汇总的完整最终正文

- 前端完成时替换可能受损的流式文本

- 补充文件 URL 终态校正回归测试

easyflow-modules/easyflow-module-agent/src/test/java/tech/easyflow/agent/runtime/AgentRunServiceDraftAndHitlTest.java

@@ -229,8 +229,14 @@ public class AgentRunServiceDraftAndHitlTest {
         Assert.assertEquals("先分析", reasoning.toString());
         Assert.assertEquals("\n最终回答", content.toString());
         Assert.assertEquals("\n最终回答", answer.toString());
-        Assert.assertTrue(emitter.envelopes.stream().anyMatch(envelope ->
-                envelope.getDomain() == ChatDomain.SYSTEM && envelope.getType() == ChatType.DONE));
+        ChatEnvelope<?> done = emitter.envelopes.stream()
+                .filter(envelope -> envelope.getDomain() == ChatDomain.SYSTEM
+                        && envelope.getType() == ChatType.DONE)
+                .findFirst()
+                .orElseThrow();
+        @SuppressWarnings("unchecked")
+        Map<String, Object> donePayload = (Map<String, Object>) done.getPayload();
+        Assert.assertEquals("\n最终回答", donePayload.get("finalText"));
     }
 
     /**