### 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_
{
        return members.count({method, param_idx}) > 0 ? Parse(arg_value) : arg_value;
    }
    
    UniValue ArgToUniValue(std::string_view arg_value, const std::string& method, const std::string& param_name)
    {
        return membersByName.count({method, param_name}) > 0 ? Parse(arg_value) : arg_value;
    }
};
CRPCConvertTable::CRPCConvertTable()
{
    for (const auto& cp : vRPCConvertParams) {
        members.emplace(cp.methodName, cp.paramIdx);
        membersByName.emplace(cp.methodName, cp.paramName);
    }
}
static CRPCConvertTable rpcCvtTable;
UniValue RPCConvertValues(const std::string &strMethod, const std::vector<std::string> &strParams)
{
    UniValue params(UniValue::VARR);
    for (unsigned int idx = 0; idx < strParams.size(); idx++) {
        std::string_view value{strParams[idx]};
        params.push_back(rpcCvtTable.ArgToUniValue(value, strMethod, idx));
    }
    return params;
}
UniValue RPCConvertNamedValues(const std::string &strMethod, const std::vector<std::string> &strParams)