fix(events): don't expand args.file for Lua callback (#31473)

Problem:  In an autocommand Lua callback whether `args.file` is expanded
          depends on whether `expand('<afile>')` has been called.
Solution: Always use the unexpanded file name for `args.file`.

Related to #31306 and vim/vim#16106. This doesn't provide `sfname`, but
at least makes `args.file` have a consistent value.

runtime/lua/vim/_meta/api.lua

@@ -956,9 +956,9 @@ function vim.api.nvim_create_augroup(name, opts) end
 ---               • event: (string) name of the triggered event
 ---                 `autocmd-events`
 ---               • group: (number|nil) autocommand group id, if any
----               • match: (string) expanded value of <amatch>
----               • buf: (number) expanded value of <abuf>
----               • file: (string) expanded value of <afile>
+---               • file: (string) <afile> (not expanded to a full path)
+---               • match: (string) <amatch> (expanded to a full path)
+---               • buf: (number) <abuf>
 ---               • data: (any) arbitrary data passed from
 ---                 `nvim_exec_autocmds()`                        *event-data*
 ---             • command (string) optional: Vim command to execute on event.