Fix sparse-packaged apps unable to discover module-specific PRI files (#6376)

* Fix sparse-packaged apps unable to discover module-specific PRI files

Sparse-packaged apps have package identity (via AddPackageByUriAsync) but
deploy resources as loose files next to the executable. GetDefaultPriFile()
determines isPackaged=true for these apps (since they have identity), causing
GetDefaultPriFileForCurentModule to pass "resources.pri" to
MrmGetFilePathFromName — which only searches for that exact filename,
skipping the [modulename].pri fallback that unpackaged apps receive.

When the "resources.pri" search fails for apps with identity, fall back to
the unpackaged discovery path (pass nullptr to MrmGetFilePathFromName) which
triggers the broader search including "[modulename].pri" derived via
PathCchRenameExtension from the process executable name.

If the fallback also fails, return the original HRESULT from the
"resources.pri" search so callers that check for specific errors
(e.g., ERROR_FILE_NOT_FOUND) are not broken.

Existing test coverage:
- DefaultResourceManagerWithExePri validates the nullptr fallback mechanism
- DefaultResourceManager validates error behavior when no PRI exists
- DefaultResourceManagerWithResourcePri validates the normal packaged path

A full sparse-app integration test requires actual sparse package
registration (AddPackageByUriAsync) which is beyond unit test scope.

Fixes: microsoft/microsoft-ui-xaml#10856

Co-authored-by: Copilot <[email protected]>

* Address PR review: simplify flow, shorten comment, gate on file-not-found

* Add backward-compatibility comment on return hr

* Move backward-compat comment to fallback block for clarity

* Keep comment sentences on single lines

* Use existing IsResourceNotFound() helper instead of manual HRESULT checks

* Merge two-line comment into single line

Agent-Logs-Url: https://github.com/microsoft/WindowsAppSDK/sessions/ed36cfc6-52ad-414b-8e1a-dc16f413a97e

Co-authored-by: yeelam-gordon <[email protected]>

---------

Co-authored-by: Copilot <[email protected]>
Co-authored-by: copilot-swe-agent[bot] <[email protected]>

Author: 3 people

dev/MRTCore/mrt/Microsoft.Windows.ApplicationModel.Resources/src/Helper.cpp

@@ -84,5 +84,19 @@ HRESULT GetDefaultPriFile(winrt::hstring& filePath)
     // GetDefaultPriFileForCurrentPackage will not handle the new case where
     // resources.pri is in the parent folder. 
     bool isPackaged = (hr != HRESULT_FROM_WIN32(APPMODEL_ERROR_NO_PACKAGE));
-    return GetDefaultPriFileForCurentModule(isPackaged, filePath);
+    hr = GetDefaultPriFileForCurentModule(isPackaged, filePath);
+
+    // Sparse-packaged apps have identity but deploy PRI files as loose files; fall back to unpackaged discovery which also searches for "[modulename].pri".
+    if (isPackaged && IsResourceNotFound(hr))
+    {
+        HRESULT hrFallback = GetDefaultPriFileForCurentModule(false, filePath);
+        if (SUCCEEDED(hrFallback))
+        {
+            hr = hrFallback;
+        }
+        // If the fallback also fails, preserve the original HRESULT (not the fallback's)
+        // for backward compatibility so callers checking for specific errors (e.g., ERROR_FILE_NOT_FOUND) are not broken.
+    }
+
+    return hr;
 }