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

: m_results{{result}}
    {
    }
    RPCResults(std::initializer_list<RPCResult> results)
        : m_results{results}
    {
    }
    /**
     * Return the description string.
     */
    std::string ToDescriptionString() const;
};
struct RPCExamples {
    const std::string m_examples;
    explicit RPCExamples(
        std::string examples)
        : m_examples(std::move(examples))
    {
    }
    std::string ToDescriptionString() const;
};
class RPCHelpMan