test: add server integration coverage for PING, ECHO, SET, and GET

commands

- Start server on an ephemeral port for tests
- Add ECHO round-trip assertion
- Verify GET on missing key returns RESP nil ($-1)

Author: Md-Talim

src/test/java/redis/integration/ServerIntegrationTest.java

@@ -0,0 +1,88 @@
+package redis.integration;
+
+import static org.assertj.core.api.Assertions.assertThat;
+
+import java.io.InputStream;
+import java.io.OutputStream;
+import java.net.ServerSocket;
+import java.net.Socket;
+import java.nio.charset.StandardCharsets;
+import org.junit.jupiter.api.AfterAll;
+import org.junit.jupiter.api.BeforeAll;
+import org.junit.jupiter.api.Test;
+import redis.Main;
+
+class ServerIntegrationTest {
+
+    private static int port;
+    private static Thread serverThread;
+
+    @BeforeAll
+    static void startServer() throws Exception {
+        try (var ss = new ServerSocket(0)) {
+            port = ss.getLocalPort();
+        }
+
+        serverThread = Thread.ofVirtual().start(() -> {
+            try {
+                Main.main(new String[] { "--port", String.valueOf(port) });
+            } catch (Exception e) {
+                // server shut down
+            }
+        });
+
+        Thread.sleep(500);
+    }
+
+    @AfterAll
+    static void stopServer() {
+        serverThread.interrupt();
+    }
+
+    private String sendCommand(String... parts) throws Exception {
+        try (var socket = new Socket("localhost", port)) {
+            OutputStream out = socket.getOutputStream();
+            InputStream in = socket.getInputStream();
+
+            StringBuilder resp = new StringBuilder();
+            resp.append("*").append(parts.length).append("\r\n");
+            for (String part : parts) {
+                resp.append("$").append(part.length()).append("\r\n");
+                resp.append(part).append("\r\n");
+            }
+
+            out.write(resp.toString().getBytes(StandardCharsets.UTF_8));
+            out.flush();
+
+            Thread.sleep(100);
+            byte[] buf = new byte[4096];
+            int len = in.read(buf);
+            return new String(buf, 0, len, StandardCharsets.UTF_8);
+        }
+    }
+
+    @Test
+    void pingReturnsPong() throws Exception {
+        String response = sendCommand("PING");
+        assertThat(response).contains("PONG");
+    }
+
+    @Test
+    void echoReturnsArgument() throws Exception {
+        String response = sendCommand("ECHO", "hello");
+        assertThat(response).contains("hello");
+    }
+
+    @Test
+    void setAndGet() throws Exception {
+        sendCommand("SET", "testkey", "testvalue");
+        String response = sendCommand("GET", "testkey");
+        assertThat(response).contains("testvalue");
+    }
+
+    @Test
+    void getMissingKeyReturnsNil() throws Exception {
+        String response = sendCommand("GET", "nonexistent_key_xyz");
+        assertThat(response).contains("$-1");
+    }
+}