### Is there an existing issue for this?

- [X] I have searched the existing issues

### Current behaviour

See inline discussion at https://github.com/bitcoin/bitcoin/pull/26467#discussion_r1269177446

@whitslack:

> Consider the case where I am paying someone on chain under the stipulation that _they_ will eat the mining fee. (Thus, I specify _their_ address in `subtractfeefrom` when I'm constructing my transaction.) Later, they tell me that they can't wait for confirmation any longer and need an urgent fee bump. If I naïvely specify their output as the `reduce_output`, I may end up paying them _**much more**_ than I intended to if I only have large UTxOs in my wallet.


### Expected behaviour

Not sure.

Improving the docs is one option, but this behaviour seem quite counter intuitive.

My own suggestion:

> Should we disable the adding of new inputs when `reduce_output` is set? If someone really intends to do that, they should probably use `outputs`.

### Steps to reproduce

Haven't tried.

### Relevant log output

_No response_

### How did you obtain Bitcoin Core

Compiled from source

### What version of Bitcoin Core are you using?

Master after #26467

### Operating system and version

Any

### Machine specifications

_No response_
pwallet->FillPSBT(psbtx, complete, SIGHASH_DEFAULT, false, true);
    const TransactionError err{pwallet->FillPSBT(psbtx, complete, SIGHASH_DEFAULT, true, false)};
    if (err != TransactionError::OK) {
        throw JSONRPCTransactionError(err);
    }
    CMutableTransaction mtx;
    complete = FinalizeAndExtractPSBT(psbtx, mtx);
    UniValue result(UniValue::VOBJ);
    const bool psbt_opt_in{options.exists("psbt") && options["psbt"].get_bool()};
    bool add_to_wallet{options.exists("add_to_wallet") ? options["add_to_wallet"].get_bool() : true};
    if (psbt_opt_in || !complete || !add_to_wallet) {
        
        CDataStream ssTx(SER_NETWORK, PROTOCOL_VERSION);
        ssTx << psbtx;
        result.pushKV("psbt", EncodeBase64(ssTx.str()));
    }
    if (complete) {
        std::string hex{EncodeHexTx(CTransaction(mtx))};
        CTransactionRef tx(MakeTransactionRef(std::move(mtx)));
        result.pushKV("txid", tx->GetHash().GetHex());
        if (add_to_wallet && !psbt_opt_in) {
            pwallet->CommitTransaction(tx, {}, {});
        } else {
            result.pushKV("hex", hex);
        }
    }
    result.pushKV("complete", complete);
    return result;
}
static void PreventOutdatedOptions(const UniValue& options)