refactor(lsp): use vim.lsp.buf_request_all internally (#34604)

runtime/lua/vim/lsp/completion.lua

@@ -430,6 +430,8 @@ function M._convert_results(
   return matches, server_start_boundary
 end
 
+-- NOTE: The reason we don't use `lsp.buf_request_all` here is because we want to filter the clients
+-- that received the request based on the trigger characters.
 --- @param clients table<integer, vim.lsp.Client> # keys != client_id
 --- @param bufnr integer
 --- @param win integer