Variable classifyACPAxonEventConst

Classifies a raw Axon event into an ACPTimelineEvent.

Classification rules:

1. SYSTEM_EVENT with a recognized type (turn.started, turn.resumed, turn.completed, turn.failed, broker.error, background.changed, ...) -> system
2. Known ACP protocol event_type (agent or client method) -> acp_protocol
3. Everything else -> unknown