refactor(protocol): map canonical tool items to legacy events (#31296)

## 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.

codex-rs/protocol/src/items.rs

@@ -13,6 +13,7 @@ use crate::parse_command::ParsedCommand;
 use crate::protocol::AgentStatus;
 use crate::protocol::CollabAgentRef;
 use crate::protocol::ExecCommandSource;
+use crate::protocol::ExecCommandStatus;
 use crate::protocol::FileChange;
 use crate::protocol::PatchApplyStatus;
 use crate::protocol::SubAgentActivityKind;
@@ -137,6 +138,16 @@ pub enum CommandExecutionStatus {
     Declined,
 }
 
+impl From<ExecCommandStatus> for CommandExecutionStatus {
+    fn from(value: ExecCommandStatus) -> Self {
+        match value {
+            ExecCommandStatus::Completed => Self::Completed,
+            ExecCommandStatus::Failed => Self::Failed,
+            ExecCommandStatus::Declined => Self::Declined,
+        }
+    }
+}
+
 #[derive(Debug, Clone, Deserialize, Serialize, TS, JsonSchema, PartialEq)]
 pub struct CommandExecutionItem {
     pub id: String,