Fix CSI controller URL for IPv6 service hosts

Build the CSI controller REST endpoint with net.JoinHostPort so IPv6 service hosts from Kubernetes environment variables are bracketed correctly while preserving IPv4 and DNS host behavior.

Add unit coverage for IPv6, IPv4, and DNS controller hosts.

Fixes #1164

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>
Signed-off-by: Maximilian Rink <github@maxrink.de>

Author: MaxRink

frontend/csi/plugin.go

@@ -5,6 +5,7 @@ package csi
 import (
 	"context"
 	"fmt"
+	"net"
 	"os"
 	"runtime"
 	"strings"
@@ -42,6 +43,10 @@ const (
 	CSIAllInOne   = "allInOne"
 )
 
+func controllerRestURL(host, port string) string {
+	return "https://" + net.JoinHostPort(host, port)
+}
+
 type Plugin struct {
 	orchestrator  core.Orchestrator
 	activatedChan chan struct{}
@@ -262,7 +267,7 @@ func NewNodePlugin(
 		hostname = tridentconfig.ServerCertName
 	}
 
-	restURL := "https://" + hostname + ":" + port
+	restURL := controllerRestURL(hostname, port)
 	p.restClient, err = controllerAPI.CreateTLSRestClient(restURL, caCert, clientCert, clientKey)
 	if err != nil {
 		return nil, err
@@ -354,7 +359,7 @@ func NewAllInOnePlugin(
 			break
 		}
 	}
-	restURL := "https://" + tridentconfig.ServerCertName + ":" + port
+	restURL := controllerRestURL(tridentconfig.ServerCertName, port)
 	p.restClient, err = controllerAPI.CreateTLSRestClient(restURL, caCert, clientCert, clientKey)
 	if err != nil {
 		return nil, err

frontend/csi/plugin_test.go

@@ -37,6 +37,40 @@ import (
 	"github.com/netapp/trident/utils/osutils"
 )
 
+func TestControllerRestURL(t *testing.T) {
+	testCases := []struct {
+		name     string
+		host     string
+		port     string
+		expected string
+	}{
+		{
+			name:     "IPv6 address",
+			host:     "2001:db8::1",
+			port:     "34571",
+			expected: "https://[2001:db8::1]:34571",
+		},
+		{
+			name:     "IPv4 address",
+			host:     "192.0.2.10",
+			port:     "34571",
+			expected: "https://192.0.2.10:34571",
+		},
+		{
+			name:     "DNS name",
+			host:     "trident-csi",
+			port:     "34571",
+			expected: "https://trident-csi:34571",
+		},
+	}
+
+	for _, tc := range testCases {
+		t.Run(tc.name, func(t *testing.T) {
+			assert.Equal(t, tc.expected, controllerRestURL(tc.host, tc.port))
+		})
+	}
+}
+
 func TestNewControllerPlugin(t *testing.T) {
 	testCases := []struct {
 		name          string