ActivityPart
data class ActivityPart(val id: String, val messageId: String, val activityType: String, val content: JsonElement) : UiPart
Structured content that is not text and not a tool call: an ACTIVITY_SNAPSHOT stream.
Modelled on activityType rather than as a purpose-built A2UI part, because that is the shape the protocol has. a2ui-surface is one value of the field; a client that understands another value can render it through the same part without this module learning about it. Interpreting an A2UI payload is the agui-a2ui module's job.