Possible to create a buy ETH-flow order

[elena-zh]

Reported in https://cowservices.slack.com/archives/C0361CDG8GP/p1761042120120369

Steps:

1. connect to Rabby+safe
2. specify a buy order with selling eth
3. change an account in Rabby to EOA

AR: order type is not changed. It is possible to proceed and place an order. But it won't be even created on the BE side

ER: it should not be possible to place buy eth-flow orders.