chore(core) Add approvals reviewer to UserTurn (#15426)

## Summary
Adds support for approvals_reviewer to `Op::UserTurn` so we can migrate
`[CodexMessageProcessor::turn_start]` to use Op::UserTurn

## Testing
- [x] Adds quick test for the new field

Co-authored-by: Codex <noreply@openai.com>

codex-rs/protocol/src/protocol.rs

@@ -255,6 +255,11 @@ pub enum Op {
         /// Policy to use for command approval.
         approval_policy: AskForApproval,
 
+        /// Reviewer to use for approval requests raised during this turn.
+        ///
+        /// When omitted, the session keeps the current setting
+        approvals_reviewer: Option<ApprovalsReviewer>,
+
         /// Policy to use for tool calls such as `local_shell`.
         sandbox_policy: SandboxPolicy,