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

desc += parser.GetChainName();
        } catch (const std::runtime_error& e) {
            desc += "error: ";
            desc += e.what();
        }
        desc += "\n";
        out_sha.Write(MakeUCharSpan(desc));
        if (out_file) {
            BOOST_REQUIRE(fwrite(desc.data(), 1, desc.size(), out_file) == desc.size());
        }
    });
    if (out_file) {
        if (fclose(out_file)) throw std::system_error(errno, std::generic_category(), "fclose failed");
        out_file = nullptr;
    }
    unsigned char out_sha_bytes[CSHA256::OUTPUT_SIZE];
    out_sha.Finalize(out_sha_bytes);
    std::string out_sha_hex = HexStr(out_sha_bytes);
    
    
    
    
    
    
    
    
    
    BOOST_CHECK_EQUAL(out_sha_hex, "f263493e300023b6509963887444c41386f44b63bc30047eb8402e8c1144854c");
}
BOOST_AUTO_TEST_CASE(util_ReadWriteSettings)