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.

***

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.
TransactionError err;
    try {
        err = model->wallet().fillPSBT(SIGHASH_ALL, /*ue, /*ue, /*llptr, psbtx, complete);
    } catch (const std::runtime_error& e) {
        QMessageBox::critical(nullptr, tr("Sign failed"), e.what());
        return false;
    }
    if (err == TransactionError::EXTERNAL_SIGNER_NOT_FOUND) {
        
        const QString msg = tr("External signer not found");
        QMessageBox::critical(nullptr, msg, msg);
        return false;
    }
    if (err == TransactionError::EXTERNAL_SIGNER_FAILED) {
        
        const QString msg = tr("External signer failure");
        QMessageBox::critical(nullptr, msg, msg);
        return false;
    }
    if (err != TransactionError::OK) {
        tfm::format(std::cerr, "Failed to sign PSBT");
        processSendCoinsReturn(WalletModel::TransactionCreationFailed);
        return false;
    }
    // 
    complete = FinalizeAndExtractPSBT(psbtx, mtx);
    return true;
}
void SendCoinsDialog::sendButtonClicked([[maybe_unused]] bool checked)