refactor(excmd): pass fargs to Lua for builtin cmds #39528

Problem:
The fallback that tokenizes `eap->arg` by unescaped whitespace (when the
parser doesn't pre-split via `EX_EXPAND` etc.) lives in `nlua_do_ucmd`,
so only user-command callbacks got `eap.fargs`. Builtin commands routed
through `nlua_call_excmd` have to re-parse the args themselves
(e.g. `M.ex_lsp`).

Solution:
- Move the tokenization into `nlua_push_eap` so every Lua handler sees
  `eap.fargs`. Keep only the `EX_NOSPC` override in `nlua_do_ucmd` (the
  `nargs=1`/`?` case which is genuinely user-command-specific).
- Drop the re-parse in `M.ex_lsp`.

test/functional/ui/messages_spec.lua

@@ -413,7 +413,7 @@ describe('ui/ext_messages', function()
           for _, chunk in ipairs(msg.content) do
             text = text .. (#chunk >= 2 and chunk[2] or chunk[1])
           end
-          t.matches('^Type number and <Enter> %(q or empty cancels%):\n', text)
+          t.matches('^Select a tag:\n', text)
           t.matches('1: > F%s+help%.txt%s+', text)
         end
         screen.messages = {}