## Description
This PR adds legacy `EventMsg` mappings for the `TurnItem` types
introduced in [#30282](https://github.com/openai/codex/pull/30282):
- `CommandExecution`
- `DynamicToolCall`
- `CollabAgentToolCall`
- `SubAgentActivity`
When their producers move to canonical `ItemStarted` / `ItemCompleted`,
raw core event consumers can still receive the existing begin/end-style
events. The canonical item lifecycle remains the live source of truth.
We also record the mapped legacy events in rollout trace so the producer
migration preserves the existing tool-runtime trace entries.
## Why
This is the compatibility layer for the follow-up producer migrations.
Splitting it out first keeps each producer PR small and keeps the legacy
mapping in one place.
## What changed
- Added `TurnItem` → legacy `EventMsg` mappings in
`protocol/src/legacy_events.rs`.
- Added the command execution status conversion used by the exec
mapping.
- Added focused coverage for command execution and dynamic tool
mappings.
Owen Lin
b9b934e99b