[PowerRename] Fix named pipe detection (#40944)

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fix/what does it change/how was it tested (even manually, if necessary)?
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## Summary of the Pull Request

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## PR Checklist

Fix a regression present on master where PowerRename is activated with
empty file list where invoked Explorer context menu.
Regression was caused by
https://github.com/microsoft/PowerToys/pull/40393

- [ ] Closes: #xxx
- [ ] **Communication:** I've discussed this with core contributors
already. If the work hasn't been agreed, this work might be rejected
- [ ] **Tests:** Added/updated and all pass
- [ ] **Localization:** All end-user-facing strings can be localized
- [ ] **Dev docs:** Added/updated
- [ ] **New binaries:** Added on the required places
- [ ] [JSON for
signing](https://github.com/microsoft/PowerToys/blob/main/.pipelines/ESRPSigning_core.json)
for new binaries
- [ ] [WXS for
installer](https://github.com/microsoft/PowerToys/blob/main/installer/PowerToysSetup/Product.wxs)
for new binaries and localization folder
- [ ] [YML for CI
pipeline](https://github.com/microsoft/PowerToys/blob/main/.pipelines/ci/templates/build-powertoys-steps.yml)
for new test projects
- [ ] [YML for signed
pipeline](https://github.com/microsoft/PowerToys/blob/main/.pipelines/release.yml)
- [ ] **Documentation updated:** If checked, please file a pull request
on [our docs
repo](https://github.com/MicrosoftDocs/windows-uwp/tree/docs/hub/powertoys)
and link it here: #xxx

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or any additional comments/features here -->
## Detailed Description of the Pull Request / Additional comments

<!-- Describe how you validated the behavior. Add automated tests
wherever possible, but list manual validation steps taken as well -->
## Validation Steps Performed

Verified that PowerRename shows file list when activated:
- From Windows 11 Explorer context menu
- From Legacy Explorer context menu
- From command line passing some file paths

src/modules/powerrename/PowerRenameUILib/PowerRenameXAML/App.xaml.cpp

@@ -126,11 +126,12 @@ void App::OnLaunched(LaunchActivatedEventArgs const&)
     }
 
     auto args = std::wstring{ GetCommandLine() };
-    
+    size_t pipePos{ args.rfind(L"\\\\.\\pipe\\") };
+
     // Try to parse command line arguments first
     std::vector<std::wstring> cmdLineFiles = ParseCommandLineArgs(args);
     
-    if (!cmdLineFiles.empty())
+    if (pipePos == std::wstring::npos && !cmdLineFiles.empty())
     {
         // Use command line arguments for UI testing
         for (const auto& filePath : cmdLineFiles)
@@ -142,12 +143,10 @@ void App::OnLaunched(LaunchActivatedEventArgs const&)
     else
     {
         // Use original pipe/stdin logic for normal operation
-        size_t pos{ args.rfind(L"\\\\.\\pipe\\") };
-
         std::wstring pipe_name;
-        if (pos != std::wstring::npos)
+        if (pipePos != std::wstring::npos)
         {
-            pipe_name = args.substr(pos);
+            pipe_name = args.substr(pipePos);
         }
 
         HANDLE hStdin;