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.
{
    CMutableTransaction txn;
    txn.vin.push_back(CTxIn(outpoint));
    if (!provider || !DummySignInput(*provider, txn.vin[0], txout, can_grind_r, coin_control)) {
        return -1;
    }
    return GetVirtualTransactionInputSize(txn.vin[0]);
}
int CalculateMaximumSignedInputSize(const CTxOut& txout, const CWallet* wallet, const CCoinControl* coin_control)
{
    const std::unique_ptr<SigningProvider> provider = wallet->GetSolvingProvider(txout.scriptPubKey);
    return CalculateMaximumSignedInputSize(txout, COutPoint(), provider.get(), wallet->CanGrindR(), coin_control);
}
TxSize CalculateMaximumSignedTxSize(const CTransaction &tx, const CWallet *wallet, const std::vector<CTxOut>& txouts, const CCoinControl* coin_control)
{
    CMutableTransaction txNew(tx);
    if (!wallet->DummySignTx(txNew, txouts, coin_control)) {
        return TxSize{-1, -1};
    }
    CTransaction ctx(txNew);
    int64_t vsize = GetVirtualTransactionSize(ctx);
    int64_t weight = GetTransactionWeight(ctx);
    return TxSize{vsize, weight};
}
TxSize CalculateMaximumSignedTxSize(const CTransaction &tx, const CWallet *wallet, const CCoinControl* coin_control)