[release/9.0-staging] Fix UNC paths (#111499)

* Fix UNC paths

If the input file was a network path then the raw path returned by
GetFinalPathByHandle may return a UNC path. If so, and if the original
path wasn't a UNC path, and the original path doesn't need normalization,
we want to use the original path.

* Use MAXPATH instead

* Update src/native/corehost/hostmisc/pal.windows.cpp

Co-authored-by: Elinor Fung <elfung@microsoft.com>

* Update src/native/corehost/hostmisc/pal.windows.cpp

Co-authored-by: Elinor Fung <elfung@microsoft.com>

---------

Co-authored-by: Andy Gocke <angocke@microsoft.com>
Co-authored-by: Elinor Fung <elfung@microsoft.com>

Author: 3 people

src/native/corehost/hostmisc/pal.windows.cpp

@@ -884,8 +884,12 @@ bool pal::realpath(pal::string_t* path, bool skip_error_logging)
                 }
             }
 
-            // Remove the \\?\ prefix, unless it is necessary or was already there
-            if (LongFile::IsExtended(str) && !LongFile::IsExtended(*path) &&
+            // Remove the UNC extended prefix (\\?\UNC\) or extended prefix (\\?\) unless it is necessary or was already there
+            if (LongFile::IsUNCExtended(str) && !LongFile::IsUNCExtended(*path) && str.length() < MAX_PATH)
+            {
+                str.replace(0, LongFile::UNCExtendedPathPrefix.size(), LongFile::UNCPathPrefix);
+            }
+            else if (LongFile::IsExtended(str) && !LongFile::IsExtended(*path) &&
                 !LongFile::ShouldNormalize(str.substr(LongFile::ExtendedPrefix.size())))
             {
                 str.erase(0, LongFile::ExtendedPrefix.size());