fix(path): nvim_get_runtime_file fails on DOS 8.3 filename #40144

Problem:
stdpath() may return a DOS 8.3 "shortened" filename, because Windows
truncates some long usernames into `6ch~N` names. Then features such
as `nvim_get_runtime_file` fail to find the file.                                         
                                                                                                                    
Analysis:
When expanding an 8.3 filename path like `C:/Users/ADMINI~1/AppData/*`,
we treat `~` as a special character and first check whether a directory
named `ADMINI~1` exists under `Users`. Since no such directory actually
exists, the expansion fails.                                                                                                    
                                                                                                                    
Solution:
Treat `~` as a literal character in `do_path_expand`. Since the `~/`
case is already handled in `gen_expand_wildcards`, any remaining `~` is
just a literal character and will later be escaped to `\~` by
`file_pat_to_reg_pat` if needed.

src/nvim/path.c

@@ -687,7 +687,9 @@ static size_t do_path_expand(garray_T *gap, const char *path, size_t wildoff, in
       s = p + 1;
     } else if (path_end >= path + wildoff
 #ifdef MSWIN
-               && vim_strchr("*?[~", (uint8_t)(*path_end)) != NULL
+               // "~" not included here, we want to treat it as literal.
+               // The "~/" case is already handled in `gen_expand_wildcards`.
+               && vim_strchr("*?[", (uint8_t)(*path_end)) != NULL
 #else
                && (vim_strchr("*?[{~$", (uint8_t)(*path_end)) != NULL
                    || (!p_fic && (flags & EW_ICASE) && mb_isalpha(utf_ptr2char(path_end))))

test/functional/api/vim_spec.lua

@@ -3756,6 +3756,23 @@ describe('API', function()
       eq(p(val[1]), vimruntime .. '/syntax/vim.vim')
       eq(p(val[2]), vimruntime .. '/ftplugin/vim.vim')
     end)
+
+    it('finds files via an 8.3 filename path #25019', function()
+      skip(not is_os('win'), 'N/A: 8.3 filenames are only available on Windows')
+      local path = 'Xtest_runtime_path'
+      mkdir_p(('%s/subdir/lua'):format(path))
+      write_file(('%s/subdir/lua/foo.lua'):format(path), '')
+      finally(function()
+        rmdir(path)
+      end)
+      local path_with_shortname =
+        p(fn.system(('for %%I in ("%s") do @echo %%~sI'):format(path), ''):gsub('\n', ''))
+      skip(path == vim.fs.basename(path_with_shortname), 'N/A: 8.3 filenames are disabled')
+      exec_lua(('vim.opt.rtp:prepend("%s/*")'):format(path_with_shortname))
+      local val = api.nvim_get_runtime_file('lua/foo.lua', true)
+      eq(1, #val)
+      eq(('%s/subdir/lua/foo.lua'):format(path_with_shortname), val[1])
+    end)
   end)
 
   describe('nvim_get_all_options_info', function()