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.
if (auto ptr_wtx = wallet.GetWalletTx(outpoint.hash)) {
            
            if (ptr_wtx->tx->vout.size() <= outpoint.n) {
                return util::Error{strprintf(_("Invalid pre-selected input %s"), outpoint.ToString())};
            }
            txout = ptr_wtx->tx->vout.at(outpoint.n);
            input_bytes = CalculateMaximumSignedInputSize(txout, &wallet, &coin_control);
        } else {
            
            if (!coin_control.GetExternalOutput(outpoint, txout)) {
                return util::Error{strprintf(_("Not found pre-selected input %s"), outpoint.ToString())};
            }
        }
        if (input_bytes == -1) {
            input_bytes = CalculateMaximumSignedInputSize(txout, outpoint, &coin_control.m_external_provider, can_grind_r, &coin_control);
        }
        
        if (coin_control.HasInputWeight(outpoint)) {
            input_bytes = GetVirtualTransactionSize(coin_control.GetInputWeight(outpoint), 0, 0);
        }
        if (input_bytes == -1) {
            return util::Error{strprintf(_("Not solvable pre-selected input %s"), outpoint.ToString())}; 
        }
        
        COutput output(outpoint, txout,  0, input_bytes,  true,  true,  true,  0,  false, coin_selection_params.m_effective_feerate);
        result.Insert(output, coin_selection_params.m_subtract_fee_outputs);
    }
    return result;
}
CoinsResult AvailableCoins(const CWallet& wallet,