Zilla crashes with `IllegalArgumentException` when an Avro payload is fetched as `json`

[vordimous]

Describe the bug
A clear and concise description of what the bug is.

To Reproduce
Steps to reproduce the behavior:

1. Go to demo repo on branch bug-repro
2. follow instructions in readme
3. See error

Expected behavior
POST and GET work for the endpoint with avro

Screenshots
If applicable, add screenshots to help explain your problem.

Zilla Environment:
Describe a docker container:

{
  "Hostname": "b42d27ca152b",
  "Domainname": "",
  "User": "",
  "AttachStdin": false,
  "AttachStdout": true,
  "AttachStderr": true,
  "ExposedPorts": {
    "61616/tcp": {},
    "7114/tcp": {},
    "7190/tcp": {},
    "8080/tcp": {}
  },
  "Tty": false,
  "OpenStdin": false,
  "StdinOnce": false,
  "Env": [
    "ZILLA_INCUBATOR_ENABLED=true",
    "KAFKA_HOST=broker",
    "KAFKA_PORT=29092",
    "API_URL=http://localhost:8080",
    "KAFKA_BOOTSTRAP_SERVER=broker:29092",
    "SCHEMA_REGISTRY_URL=http://schema-registry:8081",
    "PATH=/usr/local/sbin:/usr/local/bin:/usr/sbin:/usr/bin:/sbin:/bin",
    "ZILLA_VERSION=0.9.79"
  ],
  "Cmd": [
    "start",
    "-v",
    "-e"
  ],
  "Image": "ghcr.io/aklivity/zilla:0.9.79",
  "Volumes": null,
  "WorkingDir": "",
  "Entrypoint": [
    "/opt/zilla/zilla"
  ],
  "OnBuild": null,
  "Labels": {
    "com.docker.compose.config-hash": "6a9815f7062861457c8e6f0b18cd73b73f257cc798ef793a649c2f63d4532eb4",
    "com.docker.compose.container-number": "1",
    "com.docker.compose.depends_on": "broker:service_started:false,schema-registry:service_started:false",
    "com.docker.compose.image": "sha256:5ccb18ef0734af1255899b2d45545e9fb23aac17ddd565e39cfdb0773d633785",
    "com.docker.compose.oneoff": "False",
    "com.docker.compose.project": "mndwrk-webinar-kafka-demo",
    "com.docker.compose.project.config_files": "/Users/adanelz/Code/mndwrk-webinar-kafka-demo/docker-compose.yml",
    "com.docker.compose.project.working_dir": "/Users/adanelz/Code/mndwrk-webinar-kafka-demo",
    "com.docker.compose.service": "zilla",
    "com.docker.compose.version": "2.27.0",
    "org.opencontainers.image.ref.name": "ubuntu",
    "org.opencontainers.image.version": "22.04"
  }
}

Kafka Environment:

• Provider: Confluent
• Version: 7.2.2