Cryptic "Error: io error: failed to fill whole buffer" error

The example currently fails to run with a cryptic error message. Perhaps some more context could be added? For example, did an error occur while fetching the odohconfig from DNS, or while parsing the ODoH proxy response?

```
$ cargo run -- example.com AAAA
Finished dev [unoptimized + debuginfo] target(s) in 0.39s
Running `target/debug/odoh-client-rs example.com AAAA`
Error: io error: failed to fill whole buffer
```

Start by running `cargo run -- example.com AAAA` to reproduce the error and trace where the generic I/O message is emitted. Identify whether the failure occurs while fetching the ODoHConfig from DNS or parsing the ODoH proxy response; done means the error reports that context clearly.

Written by the indexing model from the issue text.