feat: 完善多模型配置与运行切换

将模型连接统一持久化管理，并支持 Agent 运行中切换模型以及中断后选择模型继续。补充配置校验、事务与前后端交互测试。

server/src/test/java/tech/easyflow/manuagent/config/AppPropertiesValidationTest.java

@@ -0,0 +1,72 @@
+package tech.easyflow.manuagent.config;
+
+import static org.assertj.core.api.Assertions.assertThat;
+
+import java.io.IOException;
+import java.util.List;
+import org.junit.jupiter.api.Test;
+import org.springframework.boot.context.properties.EnableConfigurationProperties;
+import org.springframework.boot.env.YamlPropertySourceLoader;
+import org.springframework.boot.test.context.runner.ApplicationContextRunner;
+import org.springframework.core.env.PropertySource;
+import org.springframework.core.io.ClassPathResource;
+import org.springframework.context.annotation.Configuration;
+
+/**
+ * 验证应用主密钥沿用本地配置文件的读取方式。
+ */
+class AppPropertiesValidationTest {
+
+    private final ApplicationContextRunner contextRunner = new ApplicationContextRunner()
+            .withUserConfiguration(PropertiesConfiguration.class)
+            .withPropertyValues(
+                    "app.data-root=file:../data",
+                    "app.dashscope-key-file=./dashscope_key.txt",
+                    "app.master-key=unit-test-master-key",
+                    "app.admin-username=admin",
+                    "app.admin-password=admin123",
+                    "app.sandbox-image=runtime:test",
+                    "app.sandbox-network=bridge",
+                    "app.run-timeout=1m");
+
+    /**
+     * 默认应用配置必须直接提供主密钥，使本地启动不依赖额外环境变量。
+     *
+     * @throws IOException application.yml 无法读取时抛出
+     */
+    @Test
+    void shouldProvideMasterKeyInApplicationConfiguration() throws IOException {
+        YamlPropertySourceLoader loader = new YamlPropertySourceLoader();
+        List<PropertySource<?>> sources = loader.load(
+                "application.yml",
+                new ClassPathResource("application.yml"));
+
+        assertThat(sources)
+                .extracting(source -> source.getProperty("app.master-key"))
+                .singleElement()
+                .isInstanceOf(String.class)
+                .asString()
+                .isNotBlank()
+                .doesNotContain("APP_MASTER_KEY");
+    }
+
+    /**
+     * 提供非空加密主密钥后，配置属性应可以正常绑定并供密钥组件使用。
+     */
+    @Test
+    void shouldBindConfiguredMasterKey() {
+        contextRunner.run(context -> {
+            assertThat(context).hasNotFailed();
+            assertThat(context.getBean(AppProperties.class).masterKey())
+                    .isEqualTo("unit-test-master-key");
+        });
+    }
+
+    /**
+     * 注册应用配置属性，复用生产环境的 Spring Boot 配置绑定流程。
+     */
+    @Configuration(proxyBeanMethods = false)
+    @EnableConfigurationProperties(AppProperties.class)
+    static class PropertiesConfiguration {
+    }
+}