refactor: use strconv.Itoa instead of string(rune()) in test

Replace string(rune(i)) with strconv.Itoa(i) in hub_concurrency_test.go
for generating client IDs. While this is test code and not a production bug,
it uses the same incorrect pattern that caused the PR #575 bug.

This ensures consistent best practices across the codebase and avoids
confusion for developers who might copy this pattern.

Related: #575

internal/websocket/hub_concurrency_test.go

@@ -1,6 +1,7 @@
 package websocket
 
 import (
+	"strconv"
 	"sync"
 	"testing"
 	"time"
@@ -24,7 +25,7 @@ func TestHubConcurrentClients(t *testing.T) {
 			client := &Client{
 				hub:  hub,
 				send: make(chan []byte, 10),
-				id:   "client-register-" + string(rune(i)),
+				id:   "client-register-" + strconv.Itoa(i),
 			}
 			hub.register <- client
 			time.Sleep(time.Microsecond)