refactor: Reduce function complexity and nesting depth

- Extract helper functions from process_chunk to reduce nesting
- Extract format_error_message from extract_changeset_error
- Split extract_error_message into smaller functions to reduce complexity
- Fixes Credo refactoring opportunities

lib/mv/membership/import/member_csv.ex

@@ -302,28 +302,15 @@ defmodule Mv.Membership.Import.MemberCSV do
     max_errors = Keyword.get(opts, :max_errors, 50)
 
     {inserted, failed, errors, _collected_error_count, truncated?} =
-      Enum.reduce(chunk_rows_with_lines, {0, 0, [], 0, false}, fn {line_number, row_map},
-                                                                  {acc_inserted, acc_failed,
-                                                                   acc_errors, acc_error_count,
-                                                                   acc_truncated?} ->
-        current_error_count = existing_error_count + acc_error_count
+      Enum.reduce(chunk_rows_with_lines, {0, 0, [], 0, false}, fn {line_number, row_map}, acc ->
+        current_error_count = existing_error_count + elem(acc, 3)
 
         case process_row(row_map, line_number, custom_field_lookup) do
           {:ok, _member} ->
-            {acc_inserted + 1, acc_failed, acc_errors, acc_error_count, acc_truncated?}
+            update_inserted(acc)
 
           {:error, error} ->
-            new_acc_failed = acc_failed + 1
-
-            # Only collect errors if under limit
-            {new_acc_errors, new_error_count, new_truncated?} =
-              if current_error_count < max_errors do
-                {[error | acc_errors], acc_error_count + 1, acc_truncated?}
-              else
-                {acc_errors, acc_error_count, true}
-              end
-
-            {acc_inserted, new_acc_failed, new_acc_errors, new_error_count, new_truncated?}
+            handle_row_error(acc, error, current_error_count, max_errors)
         end
       end)
 
@@ -397,11 +384,9 @@ defmodule Mv.Membership.Import.MemberCSV do
 
   # Extracts the first error from a changeset and converts it to a MemberCSV.Error struct
   defp extract_changeset_error(changeset, csv_line_number) do
-    case Ecto.Changeset.traverse_errors(changeset, fn {msg, opts} ->
-           Enum.reduce(opts, msg, fn {key, value}, acc ->
-             String.replace(acc, "%{#{key}}", to_string(value))
-           end)
-         end) do
+    errors = Ecto.Changeset.traverse_errors(changeset, &format_error_message/1)
+
+    case errors do
       %{email: [message | _]} ->
         # Email-specific error
         %Error{
@@ -430,6 +415,56 @@ defmodule Mv.Membership.Import.MemberCSV do
     end
   end
 
+  # Helper function to update accumulator when row is successfully inserted
+  defp update_inserted({acc_inserted, acc_failed, acc_errors, acc_error_count, acc_truncated?}) do
+    {acc_inserted + 1, acc_failed, acc_errors, acc_error_count, acc_truncated?}
+  end
+
+  # Helper function to handle row error with error count limit checking
+  defp handle_row_error(
+         {acc_inserted, acc_failed, acc_errors, acc_error_count, acc_truncated?},
+         error,
+         current_error_count,
+         max_errors
+       ) do
+    new_acc_failed = acc_failed + 1
+
+    {new_acc_errors, new_error_count, new_truncated?} =
+      collect_error_if_under_limit(
+        error,
+        acc_errors,
+        acc_error_count,
+        acc_truncated?,
+        current_error_count,
+        max_errors
+      )
+
+    {acc_inserted, new_acc_failed, new_acc_errors, new_error_count, new_truncated?}
+  end
+
+  # Helper function to collect error only if under limit
+  defp collect_error_if_under_limit(
+         error,
+         acc_errors,
+         acc_error_count,
+         acc_truncated?,
+         current_error_count,
+         max_errors
+       ) do
+    if current_error_count < max_errors do
+      {[error | acc_errors], acc_error_count + 1, acc_truncated?}
+    else
+      {acc_errors, acc_error_count, true}
+    end
+  end
+
+  # Formats error message by replacing placeholders
+  defp format_error_message({msg, opts}) do
+    Enum.reduce(opts, msg, fn {key, value}, acc ->
+      String.replace(acc, "%{#{key}}", to_string(value))
+    end)
+  end
+
   # Maps changeset error messages to appropriate Gettext messages
   defp gettext_error_message(message) when is_binary(message) do
     cond do