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.
bool sign,
        bool bip32derivs,
        size_t* n_signed,
        PartiallySignedTransaction& psbtx,
        bool& complete) override
    {
        return m_wallet->FillPSBT(psbtx, complete, sighash_type, sign, bip32derivs, n_signed);
    }
    WalletBalances getBalances() override
    {
        const auto bal = GetBalance(*m_wallet);
        WalletBalances result;
        result.balance = bal.m_mine_trusted;
        result.unconfirmed_balance = bal.m_mine_untrusted_pending;
        result.immature_balance = bal.m_mine_immature;
        result.have_watch_only = haveWatchOnly();
        if (result.have_watch_only) {
            result.watch_only_balance = bal.m_watchonly_trusted;
            result.unconfirmed_watch_only_balance = bal.m_watchonly_untrusted_pending;
            result.immature_watch_only_balance = bal.m_watchonly_immature;
        }
        return result;
    }
    bool tryGetBalances(WalletBalances& balances, uint256& block_hash) override