fix: 完善转换代理行为

backend/cmd/desktop/messagebox_test.go

@@ -3,13 +3,59 @@
 package main
 
 import (
+	"errors"
+	"syscall"
 	"testing"
 )
 
-func TestMessageBoxW_WindowsOnly(t *testing.T) {
-	messageBox("测试标题", "测试消息", MB_ICONINFORMATION)
+func withMessageBoxW(t *testing.T, fn func(hwnd, text, caption, flags uintptr) (uintptr, error)) {
+	t.Helper()
+
+	old := callMessageBoxW
+	callMessageBoxW = fn
+	t.Cleanup(func() {
+		callMessageBoxW = old
+	})
+}
+
+func TestMessageBoxW_WindowsOnly_InvalidUTF16(t *testing.T) {
+	err := messageBox("bad\x00title", "测试消息", mbIconInformation)
+	if err == nil {
+		t.Fatal("包含 NUL 字符时应该返回错误")
+	}
+}
+
+func TestMessageBoxW_WindowsOnly_SuccessIgnoresLastError(t *testing.T) {
+	withMessageBoxW(t, func(_, _, _, _ uintptr) (uintptr, error) {
+		return 1, syscall.Errno(123)
+	})
+
+	if err := messageBox("测试标题", "测试消息", mbIconInformation); err != nil {
+		t.Fatalf("MessageBoxW 返回成功时应忽略 last error: %v", err)
+	}
+}
+
+func TestMessageBoxW_WindowsOnly_FailureUsesReturnValue(t *testing.T) {
+	withMessageBoxW(t, func(_, _, _, _ uintptr) (uintptr, error) {
+		return 0, syscall.Errno(5)
+	})
+
+	err := messageBox("测试标题", "测试消息", mbIconInformation)
+	if !errors.Is(err, syscall.Errno(5)) {
+		t.Fatalf("MessageBoxW 返回 0 时应返回调用错误: %v", err)
+	}
 }
 
 func TestShowError_WindowsBranch(t *testing.T) {
+	withMessageBoxW(t, func(_, _, _, _ uintptr) (uintptr, error) {
+		return 0, syscall.Errno(5)
+	})
+
+	defer func() {
+		if recovered := recover(); recovered != nil {
+			t.Fatalf("showError 不应因 MessageBoxW 失败而 panic: %v", recovered)
+		}
+	}()
+
 	showError("测试错误", "这是一条测试错误消息")
 }