[codex exec] Add item.started and support it for command execution (#4250)

Adds a new `item.started` event to `codex exec` and implements it for
command_execution item type.

```jsonl
{"type":"session.created","session_id":"019982d1-75f0-7920-b051-e0d3731a5ed8"}
{"type":"item.completed","item":{"id":"item_0","item_type":"reasoning","text":"**Executing commands securely**\n\nI'm thinking about how the default harness typically uses \"bash -lc,\" while historically \"bash\" is what we've been using. The command should be executed as a string in our CLI, so using \"bash -lc 'echo hello'\" is optimal but calling \"echo hello\" directly feels safer. The sandbox makes sure environment variables like CODEX_SANDBOX_NETWORK_DISABLED=1 are set, so I won't ask for approval. I just need to run \"echo hello\" and correctly present the output."}}
{"type":"item.completed","item":{"id":"item_1","item_type":"reasoning","text":"**Preparing for tool calls**\n\nI realize that I need to include a preamble before making any tool calls. So, I'll first state the preamble in the commentary channel, then proceed with the tool call. After that, I need to present the final message along with the output. It's possible that the CLI will show the output inline, but I must ensure that I present the result clearly regardless. Let's move forward and get this organized!"}}
{"type":"item.completed","item":{"id":"item_2","item_type":"assistant_message","text":"Running `echo` to confirm shell access and print output."}}
{"type":"item.started","item":{"id":"item_3","item_type":"command_execution","command":"bash -lc echo hello","aggregated_output":"","exit_code":null,"status":"in_progress"}}
{"type":"item.completed","item":{"id":"item_3","item_type":"command_execution","command":"bash -lc echo hello","aggregated_output":"hello\n","exit_code":0,"status":"completed"}}
{"type":"item.completed","item":{"id":"item_4","item_type":"assistant_message","text":"hello"}}
```

codex-rs/exec/src/exec_events.rs

@@ -8,6 +8,8 @@ use ts_rs::TS;
 pub enum ConversationEvent {
     #[serde(rename = "session.created")]
     SessionCreated(SessionCreatedEvent),
+    #[serde(rename = "item.started")]
+    ItemStarted(ItemStartedEvent),
     #[serde(rename = "item.completed")]
     ItemCompleted(ItemCompletedEvent),
     #[serde(rename = "error")]
@@ -20,6 +22,12 @@ pub struct SessionCreatedEvent {
     pub session_id: String,
 }
 
+/// Payload describing the start of an existing conversation item.
+#[derive(Debug, Clone, Serialize, Deserialize, PartialEq, Eq, TS)]
+pub struct ItemStartedEvent {
+    pub item: ConversationItem,
+}
+
 /// Payload describing the completion of an existing conversation item.
 #[derive(Debug, Clone, Serialize, Deserialize, PartialEq, Eq, TS)]
 pub struct ItemCompletedEvent {
@@ -85,7 +93,8 @@ pub enum CommandExecutionStatus {
 pub struct CommandExecutionItem {
     pub command: String,
     pub aggregated_output: String,
-    pub exit_code: i32,
+    #[serde(skip_serializing_if = "Option::is_none")]
+    pub exit_code: Option<i32>,
     pub status: CommandExecutionStatus,
 }