[app-server] feat: v2 apply_patch approval flow (#6760)

This PR adds the API V2 version of the apply_patch approval flow, which
centers around `ThreadItem::FileChange`.

This PR wires the new RPC (`item/fileChange/requestApproval`, V2 only)
and related events (`item/started`, `item/completed` for
`ThreadItem::FileChange`, 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.

Similar to https://github.com/openai/codex/pull/6758, 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 a few additional fields to
`EventMsg::PatchApplyBegin` and `EventMsg::PatchApplyEnd`, but those
were fairly lightweight.

However, the `EventMsg`s emitted by core are the following:
```
1) Auto-approved (no request for approval)

- EventMsg::PatchApplyBegin
- EventMsg::PatchApplyEnd

2) Approved by user
- EventMsg::ApplyPatchApprovalRequest
- EventMsg::PatchApplyBegin
- EventMsg::PatchApplyEnd

3) Declined by user
- EventMsg::ApplyPatchApprovalRequest
- EventMsg::PatchApplyBegin
- EventMsg::PatchApplyEnd
```

For a request triggering an approval, this would result in:
```
item/fileChange/requestApproval
item/started
item/completed
```

which is different from the `ThreadItem::CommandExecution` flow
introduced in https://github.com/openai/codex/pull/6758, which does the
below and is preferable:
```
item/started
item/commandExecution/requestApproval
item/completed
```

To fix this, we leverage `TurnSummaryStore` on codex_message_processor
to store a little bit of state, allowing us to fire `item/started` and
`item/fileChange/requestApproval` whenever we receive the underlying
`EventMsg::ApplyPatchApprovalRequest`, and no-oping when we receive the
`EventMsg::PatchApplyBegin` later.

This is much less invasive than modifying the order of EventMsg within
core (I tried).

The resulting payloads:
```
{
  "method": "item/started",
  "params": {
    "item": {
      "changes": [
        {
          "diff": "Hello from Codex!\n",
          "kind": "add",
          "path": "/Users/owen/repos/codex/codex-rs/APPROVAL_DEMO.txt"
        }
      ],
      "id": "call_Nxnwj7B3YXigfV6Mwh03d686",
      "status": "inProgress",
      "type": "fileChange"
    }
  }
}
```

```
{
  "id": 0,
  "method": "item/fileChange/requestApproval",
  "params": {
    "grantRoot": null,
    "itemId": "call_Nxnwj7B3YXigfV6Mwh03d686",
    "reason": null,
    "threadId": "019a9e11-8295-7883-a283-779e06502c6f",
    "turnId": "1"
  }
}
```

```
{
  "id": 0,
  "result": {
    "decision": "accept"
  }
}
```

```
{
  "method": "item/completed",
  "params": {
    "item": {
      "changes": [
        {
          "diff": "Hello from Codex!\n",
          "kind": "add",
          "path": "/Users/owen/repos/codex/codex-rs/APPROVAL_DEMO.txt"
        }
      ],
      "id": "call_Nxnwj7B3YXigfV6Mwh03d686",
      "status": "completed",
      "type": "fileChange"
    }
  }
}
```

codex-rs/app-server-protocol/src/protocol/common.rs

@@ -438,6 +438,13 @@ server_request_definitions! {
         response: v2::CommandExecutionRequestApprovalResponse,
     },
 
+    /// Sent when approval is requested for a specific file change.
+    /// This request is used for Turns started via turn/start.
+    FileChangeRequestApproval => "item/fileChange/requestApproval" {
+        params: v2::FileChangeRequestApprovalParams,
+        response: v2::FileChangeRequestApprovalResponse,
+    },
+
     /// DEPRECATED APIs below
     /// Request to approve a patch.
     /// This request is used for Turns started via the legacy APIs (i.e. SendUserTurn, SendUserMessage).