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.
{
    CScript p2pk = GetScriptForRawPubKey(add_pubkey);
    CKeyID add_address = add_pubkey.GetID();
    CPubKey found_pubkey;
    LOCK(spk_man->cs_KeyStore);
    
    BOOST_CHECK(!spk_man->HaveWatchOnly(p2pk));
    spk_man->LoadWatchOnly(p2pk);
    BOOST_CHECK(spk_man->HaveWatchOnly(p2pk));
    
    bool is_pubkey_fully_valid = add_pubkey.IsFullyValid();
    if (is_pubkey_fully_valid) {
        BOOST_CHECK(spk_man->GetWatchPubKey(add_address, found_pubkey));
        BOOST_CHECK(found_pubkey == add_pubkey);
    } else {
        BOOST_CHECK(!spk_man->GetWatchPubKey(add_address, found_pubkey));
        BOOST_CHECK(found_pubkey == CPubKey()); 
    }
    spk_man->RemoveWatchOnly(p2pk);
    BOOST_CHECK(!spk_man->HaveWatchOnly(p2pk));
    if (is_pubkey_fully_valid) {
        BOOST_CHECK(!spk_man->GetWatchPubKey(add_address, found_pubkey));
        BOOST_CHECK(found_pubkey == add_pubkey); 
    }
}
static void PollutePubKey(CPubKey& pubkey)