fix: split ambiguous failure message in OnInstallationFaceInferred

- Separate 'no terminal object' from 'invalid pick' case
- Remove misleading '请先取端面' from surface pick failure

src/UI/WPF/ViewModels/PathEditingViewModel.cs

@@ -2552,13 +2552,25 @@ namespace NavisworksTransport.UI.WPF.ViewModels
             FaceInferToolPlugin.FaceInferred -= OnInstallationFaceInferred;
             try { NavisApplication.MainDocument.Tool.Value = Tool.Select; } catch { }
 
-            if (!result.IsValid || _assemblyTerminalObject == null)
+            if (_assemblyTerminalObject == null)
             {
                 CleanupAssemblyInstallationSelection();
                 DialogHelper.ShowStatusAndDialog(
-                    "取面失败: 未命中有效面。",
+                    "选安装面失败: 尚未捕获终点箱体。",
                     "选安装面失败",
-                    "未命中有效表面。\n请确认已先完成「取端面」，然后在终点箱体表面点击。",
+                    "尚未完成终点箱体设置。\n请先点击「捕获终点箱体」，然后「取端面」，再「选安装面」。",
+                    UpdateMainStatus,
+                    System.Windows.MessageBoxImage.Warning);
+                return;
+            }
+
+            if (!result.IsValid)
+            {
+                CleanupAssemblyInstallationSelection();
+                DialogHelper.ShowStatusAndDialog(
+                    "取面失败: 未命中有效表面。",
+                    "选安装面失败",
+                    "未命中有效表面。\n请在终点箱体的平整表面上点击。",
                     UpdateMainStatus,
                     System.Windows.MessageBoxImage.Warning);
                 return;