refactor: use WaitGroup.Go to simplify code (#16539)

### Summary

Adopt sync.WaitGroup.Go (Go 1.25) to simplify tracked goroutine
spawning. This replaces the error-prone trio of wg.Add(1), go func(),
and defer wg.Done() with a single, self-contained call.

More info: https://github.com/golang/go/issues/63796

Signed-off-by: grandpig <grandpig@outlook.com>

internal/server/variable.go

@@ -212,9 +212,7 @@ func NewVariableWatcher(store VariableStore) *VariableWatcher {
 
 // Start starts watching for variable changes
 func (w *VariableWatcher) Start(interval time.Duration) {
-	w.wg.Add(1)
-	go func() {
-		defer w.wg.Done()
+	w.wg.Go(func() {
 		ticker := time.NewTicker(interval)
 		defer ticker.Stop()
 
@@ -228,7 +226,7 @@ func (w *VariableWatcher) Start(interval time.Duration) {
 				return
 			}
 		}
-	}()
+	})
 	common.Info("Variable watcher started", zap.Duration("interval", interval))
 }