fix(path): DOS device path check #40976

Problem:
The current check is intended to match DOS device paths starting with
`\\?` or `\\.` but will match any path starting with `?` or `.`,
including relative paths such as `.\`.
This is because the leading slashes are removed before the comparison.

Solution:
Changes the check to include the path's prefix, assuring the path has
the correct amount of separators for a DOS device path.

src/nvim/os/fs.c

@@ -1227,7 +1227,7 @@ bool os_fileinfo2(const char *path, FileInfo *info)
     info->rest_off = (size_t)(p - path);
     return true;
   }
-  if (p[0] == '?' || p[0] == '.') {
+  if (leading_slashes >= 2 && (p[0] == '?' || p[0] == '.')) {
     if (!vim_ispathsep_nocolon(p[1])) {
       return true;
     }