Vereinfacht client: email normalization, multi-match warning, Bypass tests, doc note

- Normalize email (trim + downcase) before filter lookup
- Log warning when API returns multiple contacts for same email
- Add Bypass tests for find_contact_by_email (query params, empty/single response parsing)
- Document vereinfacht_required_field? as legacy/unused in vereinfacht-api.md
- Add bypass dependency (dev+test) for HTTP stubbing

lib/mv/vereinfacht/client.ex

@@ -183,8 +183,9 @@ defmodule Mv.Vereinfacht.Client do
   end
 
   defp do_find_contact_by_email(email) do
+    normalized_email = email |> String.trim() |> String.downcase()
     base = base_url() |> String.trim_trailing("/") |> then(&"#{&1}/finance-contacts")
-    encoded_email = URI.encode_www_form(email |> String.trim())
+    encoded_email = URI.encode_www_form(normalized_email)
     url = "#{base}?filter[isExternal]=true&filter[email]=#{encoded_email}"
 
     case Req.get(url, [headers: headers(api_key())] ++ req_http_options()) do
@@ -202,11 +203,19 @@ defmodule Mv.Vereinfacht.Client do
     end
   end
 
-  defp get_first_contact_id_from_list(%{"data" => [%{"id" => id} | _]}) do
-    normalize_contact_id(id)
+  defp get_first_contact_id_from_list(%{"data" => data} = _body) when is_list(data) do
+    if length(data) > 1 do
+      Logger.warning(
+        "Vereinfacht find_contact_by_email: API returned multiple contacts for same email (count: #{length(data)}), using first. Check for duplicate or inconsistent data."
+      )
+    end
+
+    case data do
+      [%{"id" => id} | _] -> normalize_contact_id(id)
+      [] -> nil
+    end
   end
 
-  defp get_first_contact_id_from_list(%{"data" => []}), do: nil
   defp get_first_contact_id_from_list(_), do: nil
 
   defp normalize_contact_id(id) when is_binary(id), do: id