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

{
    CHash256 out_sha;
    FILE* out_file = nullptr;
    if (const char* out_path = getenv("ARGS_MERGE_TEST_OUT")) {
        out_file = fsbridge::fopen(out_path, "w");
        if (!out_file) throw std::system_error(errno, std::generic_category(), "fopen failed");
    }
    ForEachMergeSetup([&](const ActionList& arg_actions, const ActionList& conf_actions, bool soft_set, bool force_set,
                          const std::string& section, const std::string& network, bool net_specific) {
        TestArgsManager parser;
        LOCK(parser.cs_args);
        std::string desc = "net=";