A user reported to me that they are unable to create a PSBT with a legacy watchonly wallet when they use the GUI. They get an error "The amount exceeds your balance".

I think the issue is that the GUI will retrieve the wallet's balance via `AvailableCoins` before calling `CreateTransaction`. This call to `AvailableCoins` is resulting in fewer coins being returned with recent changes to how `AvailableCoins` works, particularly with additional filtering of spendable coins and preset coins.

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Note that this does not effect descriptor wallets. A workaround for this issue is to switch to using descriptor wallets. An existing legacy wallet can be turned into a descriptor wallet using the `migratewallet` RPC. However many watchonly legacy wallets are likely the result of importing descriptors via `importmulti`. For such wallets, I recommend creating a new descriptor wallet and re-importing the descriptors.
{
    fHaveWatchOnly = fHaveWatchonly;
    Q_EMIT notifyWatchonlyChanged(fHaveWatchonly);
}
bool WalletModel::validateAddress(const QString& address) const
{
    return IsValidDestinationString(address.toStdString());
}
WalletModel::SendCoinsReturn WalletModel::prepareTransaction(WalletModelTransaction &transaction, const CCoinControl& coinControl)
{
    CAmount total = 0;
    bool fSubtractFeeFromAmount = false;
    QList<SendCoinsRecipient> recipients = transaction.getRecipients();
    std::vector<CRecipient> vecSend;
    if(recipients.empty())
    {
        return OK;
    }
    QSet<QString> setAddress; 
    int nAddresses = 0;
    
    for (const SendCoinsRecipient &rcp : recipients)
    {
        if (rcp.fSubtractFeeFromAmount)
            fSubtractFeeFromAmount = true;
        {   
            if(!validateAddress(rcp.address))