test(gateway): bind bootstrap API and public documentation

zuul-gateway/src/main/java/com/xtrmetl/gateway/ZuulGatewayApplication.java

@@ -4,9 +4,18 @@
 import org.springframework.boot.autoconfigure.SpringBootApplication;
 import org.springframework.cloud.client.discovery.EnableDiscoveryClient;
 
+/**
+ * Starts the standalone mightyETL gateway with service discovery during gateway bootstrap.
+ */
 @SpringBootApplication
 @EnableDiscoveryClient
 public class ZuulGatewayApplication {
+
+    /**
+     * Launches the gateway through Spring Boot.
+     *
+     * @param args command-line arguments passed to Spring Boot
+     */
     public static void main(String[] args) {
         SpringApplication.run(ZuulGatewayApplication.class, args);
     }

zuul-gateway/src/test/java/com/xtrmetl/gateway/ZuulGatewayApplicationContractTest.java

@@ -0,0 +1,48 @@
+package com.xtrmetl.gateway;
+
+import org.junit.jupiter.api.Test;
+import org.springframework.boot.autoconfigure.SpringBootApplication;
+import org.springframework.cloud.client.discovery.EnableDiscoveryClient;
+
+import java.io.IOException;
+import java.lang.reflect.Method;
+import java.lang.reflect.Modifier;
+import java.nio.charset.StandardCharsets;
+import java.nio.file.Files;
+import java.nio.file.Path;
+
+import static org.junit.jupiter.api.Assertions.assertNotNull;
+import static org.junit.jupiter.api.Assertions.assertTrue;
+
+/**
+ * Verifies the standalone gateway bootstrap contract and its beginner-readable public documentation.
+ */
+class ZuulGatewayApplicationContractTest {
+
+    @Test
+    void bootstrapClassKeepsTheRequiredGatewayAnnotations() {
+        assertNotNull(ZuulGatewayApplication.class.getAnnotation(SpringBootApplication.class));
+        assertNotNull(ZuulGatewayApplication.class.getAnnotation(EnableDiscoveryClient.class));
+    }
+
+    @Test
+    void mainEntryPointRemainsPublicStatic() throws NoSuchMethodException {
+        Method main = ZuulGatewayApplication.class.getDeclaredMethod("main", String[].class);
+
+        assertTrue(Modifier.isPublic(main.getModifiers()));
+        assertTrue(Modifier.isStatic(main.getModifiers()));
+    }
+
+    @Test
+    void publicBootstrapApiHasBeginnerReadableJavadoc() throws IOException {
+        String source = Files.readString(
+                Path.of("src/main/java/com/xtrmetl/gateway/ZuulGatewayApplication.java"),
+                StandardCharsets.UTF_8
+        );
+
+        assertTrue(source.contains("Starts the standalone mightyETL gateway"));
+        assertTrue(source.contains("service discovery during gateway bootstrap"));
+        assertTrue(source.contains("Launches the gateway through Spring Boot"));
+        assertTrue(source.contains("@param args command-line arguments passed to Spring Boot"));
+    }
+}