feat: minimum_balance for placing orders set to 1 atom for FOK

[alfetopito]

Description

The frontend wants to enable placement of LIMIT orders without requiring the full balance.
It can be tested here cowprotocol/cowswap#3742

It is already possible for Partially fillable orders due to the partial fill nature, but Fill or kill orders still require the full balance.

The arguments for the feature is:

1. allow users setting up a limit order in advance of receiving funds, for example in a newly created Safe. Once funds are received, order becomes valid and trade-able

The argument to remove the Fill or kill limitation is:

1. allow users a seamless experience when placing LIMIT orders across both sub-types, without requiring a different balance constraint

Due to spam protection, 1 atom of the sell token balance is still required, and that is not being removed.

Changes

• Remove full balance limitation for both order types (Fill or kill and Partially fillable)
• Turn minimum_balance function into a constant of value 1.

How to test

This change relies on existing unit tests

Notes

Please do let me know where best to put the constant, whether there is need to add any specific unit test and so on

[MartinquaXD]

An e2e test would be nice:

1. create order where user only has 1 wei of the sell token
2. fund the user account
3. await order getting settled

But I can also add that to the PR if you like.

[alfetopito]

An e2e test would be nice:

1. create order where user only has 1 wei of the sell token

2. fund the user account

3. await order getting settled

That's way beyond my current capacity and patience levels

But I can also add that to the PR if you like.

Pretty please? 🥺

[squadgazzz]

LG