The hello response sent by mongocryptd has the quirk that it doesn't include the maxWriteBatchSize field; in practice this isn't a real issue because insert is never sent to mongocryptd, but the field is otherwise required for all server versions.

To work around this, the Rust driver defaults the field to 100,000 (the value currently sent by servers) if it's not set. Once mongocryptd support is removed entirely in favor of crypt_shared, this defaulting should be removed.