ConversationEvent.harness_config should be bytes

AX has no opinionation whether the config can be represented as a struct type. The config serialization and deserialization is a `Harness` implementation detail.

The config could be:
- A struct or a list serialized as JSON
- A proto payload
- A yaml payload
- A payload in a custom configuration format

Start by locating ConversationEvent and the Harness implementation that owns config serialization and deserialization. Trace how harness_config is produced and consumed, then verify that representing it as bytes supports struct, JSON, proto, YAML, and custom payloads without imposing a format.

Written by the indexing model from the issue text.