feat: 支持配置 system 消息内容格式

- 为 OpenAI 兼容模型增加字符串与文本数组两种 system content 策略

- 在 AgentScope 模型工厂统一安装兼容 formatter 并补充测试

easy-agents-agent-runtime/src/test/java/com/easyagents/agent/runtime/agentscope/AgentOpenAIChatFormatterTest.java

@@ -0,0 +1,62 @@
+package com.easyagents.agent.runtime.agentscope;
+
+import com.easyagents.agent.runtime.model.AgentGenerationOptions;
+import com.easyagents.agent.runtime.model.AgentModelProviderType;
+import com.easyagents.agent.runtime.model.AgentModelSpec;
+import com.easyagents.agent.runtime.model.AgentSystemContentFormat;
+import io.agentscope.core.formatter.openai.dto.OpenAIContentPart;
+import io.agentscope.core.formatter.openai.dto.OpenAIMessage;
+import io.agentscope.core.message.Msg;
+import io.agentscope.core.message.MsgRole;
+import io.agentscope.core.model.OpenAIChatModel;
+import org.junit.Assert;
+import org.junit.Test;
+
+import java.lang.reflect.Field;
+import java.util.List;
+
+/**
+ * Agent OpenAI Chat 消息格式兼容测试。
+ */
+public class AgentOpenAIChatFormatterTest {
+
+    /**
+     * 验证 system 文本转换为 text 内容块数组，普通 user 文本保持字符串。
+     */
+    @Test
+    public void shouldConvertOnlySystemTextToContentParts() {
+        AgentOpenAIChatFormatter formatter = new AgentOpenAIChatFormatter();
+        List<OpenAIMessage> messages = formatter.format(List.of(
+                Msg.builder().role(MsgRole.SYSTEM).textContent("system prompt").build(),
+                Msg.builder().role(MsgRole.USER).textContent("hello").build()));
+
+        Assert.assertTrue(messages.get(0).getContent() instanceof List<?>);
+        List<OpenAIContentPart> contentParts = messages.get(0).getContentAsList();
+        Assert.assertEquals(1, contentParts.size());
+        Assert.assertEquals("text", contentParts.get(0).getType());
+        Assert.assertEquals("system prompt", contentParts.get(0).getText());
+        Assert.assertEquals("hello", messages.get(1).getContent());
+    }
+
+    /**
+     * 验证内容块数组策略会安装 EasyAgents 的 OpenAI Formatter。
+     *
+     * @throws Exception 反射读取 Formatter 失败时抛出
+     */
+    @Test
+    public void textPartsPolicyShouldInstallAgentOpenAIFormatter() throws Exception {
+        AgentModelSpec spec = new AgentModelSpec();
+        spec.setProviderType(AgentModelProviderType.OPENAI_COMPATIBLE);
+        spec.setModelName("vlm-test");
+        spec.setBaseUrl("http://model.example.com/v1");
+        spec.setApiKey("test-key");
+        spec.setSystemContentFormat(AgentSystemContentFormat.TEXT_PARTS);
+
+        OpenAIChatModel model = (OpenAIChatModel) new AgentScopeModelFactory()
+                .create(spec, new AgentGenerationOptions());
+        Field formatterField = OpenAIChatModel.class.getDeclaredField("formatter");
+        formatterField.setAccessible(true);
+
+        Assert.assertTrue(formatterField.get(model) instanceof AgentOpenAIChatFormatter);
+    }
+}