values smaller than 0.001 sat/B can't be represented by CFeeRate, but they are not rejected.

```
./src/bitcoin-cli -named sendtoaddress address=bcrt1qwkmhqum095zau5rf2velq55cq938vrd7uw8m4r amount=1.23 fee_rate=0.0009

error code: -6
error message:
Fee rate (0.000 sat/vB) is lower than the minimum fee rate setting (1.000 sat/vB)
```

```
./src/bitcoin-cli -named sendtoaddress address=bcrt1qwkmhqum095zau5rf2velq55cq938vrd7uw8m4r amount=1.23 fee_rate=0.000000000009

error code: -3
error message:
Invalid amount

{
    return RPCHelpMan{"listwallets",
                "Returns a list of currently loaded wallets.\n"
                "For full information on the wallet, use \"getwalletinfo\"\n",
                {},
                RPCResult{
                    RPCResult::Type::ARR, "", "",
                    {
                        {RPCResult::Type::STR, "walletname", "the wallet name"},
                    }
                },
                RPCExamples{
                    HelpExampleCli("listwallets", "")
            + HelpExampleRpc("listwallets", "")
                },
        [&](const RPCHelpMan& self, const JSONRPCRequest& request) -> UniValue
{
    UniValue obj(UniValue::VARR);
    for (const std::shared_ptr<CWallet>& wallet : GetWallets()) {
        LOCK(wallet->cs_wallet);
        obj.push_back(wallet->GetName());
    }
    return obj;
},
    };
}
static RPCHelpMan loadwallet()