refactor: reduce nesting depth and improve code readability

- Replace Enum.map |> Enum.join with Enum.map_join for efficiency
- Extract helper functions to reduce nesting depth from 4 to 2
- Rename is_current_cycle? to current_cycle? following Elixir conventions

test/membership_fees/changes/validate_same_interval_test.exs

@@ -215,8 +215,7 @@ defmodule Mv.MembershipFees.Changes.ValidateSameIntervalTest do
     defp extract_error_message(%Ash.Error.Invalid{errors: errors}) do
       errors
       |> Enum.filter(&(&1.field == :membership_fee_type_id))
-      |> Enum.map(& &1.message)
-      |> Enum.join(" ")
+      |> Enum.map_join(" ", & &1.message)
     end
   end
 end