Investigate creating AIContent-types for Workflow Events

When hosting Workflow-as-Agent we surface `WorkflowEvent` instances that do not have corresponding `AIContent` types as empty `ChatMessage` objects with `RawRepresentation` set to the `WorkflowEvent`, but this results in odd-looking messages when debugging: `[agent: null]`.

We should consider making the set of `AIContent` types richer, supporting `WorkflowEvent` types.

Start by tracing how WorkflowEvent instances are represented as AIContent and empty ChatMessage objects, including the RawRepresentation path. Compare the existing AIContent types with the workflow event cases described in the issue, then define the supported event mappings and clearer debugging output. Done means WorkflowEvent types have an agreed AIContent representation without the current odd-looking messages.

Written by the indexing model from the issue text.