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/deepdoc/parser/pdf/parser_test.go

@@ -565,16 +565,14 @@ func TestParser_ConcurrentSafety(t *testing.T) {
 	var wg sync.WaitGroup
 	n := 8
 	for range n {
-		wg.Add(1)
-		go func() {
-			defer wg.Done()
+		wg.Go(func() {
 			for range 5 {
 				eng := &MockEngine{NumPages: 2}
 				if _, err := p.ParseRaw(context.Background(), eng, mockDLA); err != nil {
 					t.Errorf("ParseRaw: %v", err)
 				}
 			}
-		}()
+		})
 	}
 	wg.Wait()
 }