Helper contract doesn't respect MAX_IN_RATIO

[fleupold]

Currently, the helper contract may suggest orders that are later on rejected by the verification logic (cf. this settlement).

This is due to certain invariants in the verify function, e.g.

cow-amm/src/contracts/BCoWPool.sol

Lines 124 to 127 in f3f77d1

	uint256 buyTokenBalance = order.buyToken.balanceOf(address(this));
	if (order.buyAmount > bmul(buyTokenBalance, MAX_IN_RATIO)) {
	revert BPool_TokenAmountInAboveMaxRatio();
	}

Expected behavior

The helper contract only suggests orders which would later on pass verification. In this case the limit amount should be scaled down to comply with MAX_IN_RATIO.

[sunce86]

Related to cowprotocol/services#3102