fix: Endpoint#toString host formatting; add Endpoint unit test

sentinel-transport/sentinel-transport-common/src/main/java/com/alibaba/csp/sentinel/transport/endpoint/Endpoint.java

@@ -44,6 +44,6 @@ public void setPort(int port) {
 
     @Override
     public String toString() {
-        return "Endpoint{" + "protocol=" + protocol + ", host='" + host + ", port=" + port + '}';
+        return "Endpoint{" + "protocol=" + protocol + ", host='" + host + '\'' + ", port=" + port + '}';
     }
 }

sentinel-transport/sentinel-transport-common/src/test/java/com/alibaba/csp/sentinel/transport/endpoint/EndpointTest.java

@@ -0,0 +1,32 @@
+package com.alibaba.csp.sentinel.transport.endpoint;
+
+import static org.junit.Assert.assertEquals;
+
+import org.junit.Test;
+
+public class EndpointTest {
+    @Test
+    public void testToStringIPv4() {
+        Endpoint e = new Endpoint(Protocol.HTTP, "127.0.0.1", 8080);
+        assertEquals("Endpoint{protocol=HTTP, host='127.0.0.1', port=8080}", e.toString());
+    }
+
+    @Test
+    public void testToStringIPv6() {
+        Endpoint e = new Endpoint(Protocol.HTTP, "fe80::1", 8080);
+        // Endpoint#toString doesn't modify the host, so IPv6 remains unbracketed
+        assertEquals("Endpoint{protocol=HTTP, host='fe80::1', port=8080}", e.toString());
+    }
+
+    @Test
+    public void testToStringAlreadyBracketed() {
+        Endpoint e = new Endpoint(Protocol.HTTPS, "[fe80::2]", 443);
+        assertEquals("Endpoint{protocol=HTTPS, host='[fe80::2]', port=443}", e.toString());
+    }
+
+    @Test
+    public void testToStringNullHost() {
+        Endpoint e = new Endpoint(Protocol.HTTP, null, 0);
+        assertEquals("Endpoint{protocol=HTTP, host='null', port=0}", e.toString());
+    }
+}