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.

src/nvim/api/autocmd.c

@@ -386,9 +386,9 @@ cleanup:
 ///                 - id: (number) autocommand id
 ///                 - 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. Cannot be used with
 ///             {callback}