fix(lsp): suppress completion request if completion is active (#30028)

Problem: the autotrigger mechanism could fire completion requests despite
completion already being active from another completion mechanism or manual
trigger

Solution: add a condition to avoid an additional request.

runtime/lua/vim/lsp/completion.lua

@@ -403,6 +403,10 @@ local function trigger(bufnr, clients)
   reset_timer()
   Context:cancel_pending()
 
+  if tonumber(vim.fn.pumvisible()) == 1 and Context.isIncomplete then
+    return
+  end
+
   local win = api.nvim_get_current_win()
   local cursor_row, cursor_col = unpack(api.nvim_win_get_cursor(win)) --- @type integer, integer
   local line = api.nvim_get_current_line()