fix financial_transactions controller to respond with 422 for invalid params

[yksflip]

this is a follow up for a bug discovered in #969

      # TODO: fix controller to actually send a 422 for invalid params?
      # Expected response code '200' to match '422'
      # Response body: {"financial_transaction":{"id":316,"user_id":599,"user_name":"Lisbeth ","amount":-3.0,"note":"-2","created_at":"2022-12-12T13:05:32.000+01:00","financial_transaction_type_id":346,"financial_transaction_type_name":"aut est iste #9"}}
      #
      # response '422', 'invalid parameter value' do
      #   # schema '$ref' => '#/components/schemas/Error422'
      #   let(:financial_transaction) { { amount: -3, financial_transaction_type_id: create(:financial_transaction_type).id, note: -2 } }
      #   run_test!