feat: 打通智能体图片媒体运行链路

- 增加稳定媒体引用解析与模型图片能力校验

- 修复图片 Data URI 与模型完整文本读取

- 补充媒体解析和消息兼容测试

easy-agents-support/src/main/java/com/easyagents/flow/support/provider/EasyAgentsLlm.java

@@ -61,7 +61,7 @@ public class EasyAgentsLlm implements Llm {
 
         AiMessage aiMessage = response.getMessage();
         if (aiMessage != null) {
-            return aiMessage.getContent();
+            return aiMessage.getTextContent();
         }
 
         throw new RuntimeException("EasyAgentsLlm can not get aiMessage!");