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

@@ -963,9 +963,9 @@ function vim.api.nvim_create_augroup(name, opts) end
 ---     - 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}