adk-2.0 producer: emit AGENT_TRANSFER from actions.transfer_to_agent

Parent tracker: #190 (v15 contract).
Wave: 2 — Producer event rows.
Blocked by: #194 (EventData source-event plumbing).

## Contract

In `on_event_callback`, when `event.actions.transfer_to_agent` is non-null, emit an `AGENT_TRANSFER` event:

- Required content: `{from_agent, to_agent}`. The ADK field is just the target agent name string.
- `reason` is **optional** — include only when it can be inferred from a co-occurring tool result or message; never invent.

## Acceptance

- [ ] `AGENT_TRANSFER` emitted exactly once per `transfer_to_agent` action.
- [ ] `from_agent` is the agent the event came from; `to_agent` is the target.
- [ ] `reason` is omitted (not synthesized) when no co-occurring signal provides it.
- [ ] Multi-agent fixture verifies a transfer-chain produces a sequence of `AGENT_TRANSFER` rows queryable via `agent_transfer_chains` (C3).

## References

- #190 (v15 producer §5).
- `src/google/adk/events/event_actions.py` (`transfer_to_agent`).
- `src/google/adk/plugins/bigquery_agent_analytics_plugin.py:227` (existing `TransferToAgentTool` tool_origin tagging).