[app-server] feat: add v2 command execution approval flow (#6758)

This PR adds the API V2 version of the command‑execution approval flow
for the shell tool.

This PR wires the new RPC (`item/commandExecution/requestApproval`, V2
only) and related events (`item/started`, `item/completed`, and
`item/commandExecution/delta`, which are emitted in both V1 and V2)
through the app-server
protocol. The new approval RPC is only sent when the user initiates a
turn with the new `turn/start` API so we don't break backwards
compatibility with VSCE.

The approach I took was to make as few changes to the Codex core as
possible, leveraging existing `EventMsg` core events, and translating
those in app-server. I did have to add additional fields to
`EventMsg::ExecCommandEndEvent` to capture the command's input so that
app-server can statelessly transform these events to a
`ThreadItem::CommandExecution` item for the `item/completed` event.

Once we stabilize the API and it's complete enough for our partners, we
can work on migrating the core to be aware of command execution items as
a first-class concept.

**Note**: We'll need followup work to make sure these APIs work for the
unified exec tool, but will wait til that's stable and landed before
doing a pass on app-server.

Example payloads below:
```
{
  "method": "item/started",
  "params": {
    "item": {
      "aggregatedOutput": null,
      "command": "/bin/zsh -lc 'touch /tmp/should-trigger-approval'",
      "cwd": "/Users/owen/repos/codex/codex-rs",
      "durationMs": null,
      "exitCode": null,
      "id": "call_lNWWsbXl1e47qNaYjFRs0dyU",
      "parsedCmd": [
        {
          "cmd": "touch /tmp/should-trigger-approval",
          "type": "unknown"
        }
      ],
      "status": "inProgress",
      "type": "commandExecution"
    }
  }
}
```

```
{
  "id": 0,
  "method": "item/commandExecution/requestApproval",
  "params": {
    "itemId": "call_lNWWsbXl1e47qNaYjFRs0dyU",
    "parsedCmd": [
      {
        "cmd": "touch /tmp/should-trigger-approval",
        "type": "unknown"
      }
    ],
    "reason": "Need to create file in /tmp which is outside workspace sandbox",
    "risk": null,
    "threadId": "019a93e8-0a52-7fe3-9808-b6bc40c0989a",
    "turnId": "1"
  }
}
```

```
{
  "id": 0,
  "result": {
    "acceptSettings": {
      "forSession": false
    },
    "decision": "accept"
  }
}
```

```
{
  "params": {
    "item": {
      "aggregatedOutput": null,
      "command": "/bin/zsh -lc 'touch /tmp/should-trigger-approval'",
      "cwd": "/Users/owen/repos/codex/codex-rs",
      "durationMs": 224,
      "exitCode": 0,
      "id": "call_lNWWsbXl1e47qNaYjFRs0dyU",
      "parsedCmd": [
        {
          "cmd": "touch /tmp/should-trigger-approval",
          "type": "unknown"
        }
      ],
      "status": "completed",
      "type": "commandExecution"
    }
  }
}
```

codex-rs/app-server/src/codex_message_processor.rs

@@ -1245,7 +1245,7 @@ impl CodexMessageProcessor {
                 // Auto-attach a conversation listener when starting a thread.
                 // Use the same behavior as the v1 API with experimental_raw_events=false.
                 if let Err(err) = self
-                    .attach_conversation_listener(conversation_id, false)
+                    .attach_conversation_listener(conversation_id, false, ApiVersion::V2)
                     .await
                 {
                     tracing::warn!(
@@ -1523,7 +1523,7 @@ impl CodexMessageProcessor {
             }) => {
                 // Auto-attach a conversation listener when resuming a thread.
                 if let Err(err) = self
-                    .attach_conversation_listener(conversation_id, false)
+                    .attach_conversation_listener(conversation_id, false, ApiVersion::V2)
                     .await
                 {
                     tracing::warn!(
@@ -2376,7 +2376,7 @@ impl CodexMessageProcessor {
             experimental_raw_events,
         } = params;
         match self
-            .attach_conversation_listener(conversation_id, experimental_raw_events)
+            .attach_conversation_listener(conversation_id, experimental_raw_events, ApiVersion::V1)
             .await
         {
             Ok(subscription_id) => {
@@ -2417,6 +2417,7 @@ impl CodexMessageProcessor {
         &mut self,
         conversation_id: ConversationId,
         experimental_raw_events: bool,
+        api_version: ApiVersion,
     ) -> Result<Uuid, JSONRPCErrorError> {
         let conversation = match self
             .conversation_manager
@@ -2440,6 +2441,7 @@ impl CodexMessageProcessor {
 
         let outgoing_for_task = self.outgoing.clone();
         let pending_interrupts = self.pending_interrupts.clone();
+        let api_version_for_task = api_version;
         tokio::spawn(async move {
             loop {
                 tokio::select! {
@@ -2495,6 +2497,7 @@ impl CodexMessageProcessor {
                             conversation.clone(),
                             outgoing_for_task.clone(),
                             pending_interrupts.clone(),
+                            api_version_for_task,
                         )
                         .await;
                     }