Make URL construction IPv6-compatible

While browsing the code, I spotted this line that would break if the
IP address was IPv6. Use the Go standard library helper to safely join
a host and port, which knows how to handle IPv6 properly.

Signed-off-by: Russell Bryant <rbryant@redhat.com>

Author: russellb

pkg/scheduling/pd/scheduler.go

@@ -4,6 +4,7 @@ import (
 	"context"
 	"errors"
 	"fmt"
+	"net"
 	"time"
 
 	"sigs.k8s.io/controller-runtime/pkg/log"
@@ -133,7 +134,8 @@ func (s *Scheduler) Schedule(ctx context.Context, req *types.LLMRequest) (*types
 		}
 
 		// TODO: should the scheme be conifgurable (e.g., https://)?
-		prefillURL := fmt.Sprintf("http://%s:%d", prefillRes.TargetPod.GetPod().Address, pool.Spec.TargetPortNumber)
+		prefillURL := "http://" + net.JoinHostPort(prefillRes.TargetPod.GetPod().Address, fmt.Sprint(pool.Spec.TargetPortNumber))
+
 		if req.Headers == nil { // TODO should always be populated?
 			req.Headers = make(map[string]string)
 		}