Fail to set safe threshold to targetThreshold

Summary

There is no guarantee that the signerCount value of HSG is in sync with the number of valid owners of the safe when owner calls setTargetThreshold() to change the target threshold of the HSG. This will be the case if the owner does not call reconcileSignerCount() beforehand or during set up of HSG that is being wired to an underlying safe.

This means the function _setSafeThreshold() may not be called and the value of threshold on the underlying safe remains higher than targetThreshold on the HSG.

Vulnerability Detail

This problem is made worse by the behaviour of reconcileSignerCount() which does not update the safe threshold if validSignerCount > targetThreshold && safe.getThreshold() > targetThresold.

Impact

The value of threshold on the safe may remain higher than targetThreshold on the HSG for a long period of time and go unnoticed to users / owner. This may result in safe tx execution failing unexpectedly.

After safe tx execution checkAfterExecution() will call _getCorrectThreshold() which will return the value of targetThreshold if the number of valid signers is equal to or higher than targetThreshold. The check if (safe.getThreshold() != _getCorrectThreshold()) { revert ... } will then revert the execution of the transaction.

I believe the impact is high as this issue is likely to arise without being caught and create confusion to users when they try to execute their transaction.