fix: preserve auto review across config and delegation (#26230)

## Why

Auto Review should remain the effective approval reviewer when settings
cross runtime boundaries. A config or app-server round trip must not
change the reviewer identity, and delegated work must not silently fall
back to user review.

This requires both a stable canonical serialized value and propagation
of the effective setting. `auto_review` is the canonical value across
protocol and app-server output, while `guardian_subagent` remains
accepted as backward-compatible input.

## What changed

- serialize `ApprovalsReviewer::AutoReview` consistently as
`auto_review` across core protocol and app-server v2
- continue accepting `guardian_subagent` when reading existing config or
client requests
- carry the active turn's approval reviewer into spawned agents
- update config/debug expectations and add delegated-task regression
coverage

## Scope

This does not change Guardian policy or remove compatibility with
existing `guardian_subagent` inputs. It preserves the selected reviewer
across serialization, config reloads, app-server settings, and delegated
task setup.

Related Guardian changes are split independently:

- #26231 adds denials and soft denials
- #26334 retries transient reviewer failures
- #26333 reuses narrowly scoped low-risk approvals
- #26232 adds TUI denial recovery

## Validation

- `just test -p codex-app-server-protocol` (224 passed)
- regression coverage for delegated task reviewer propagation
- serialization coverage for canonical `auto_review` output and legacy
`guardian_subagent` input

---------

Co-authored-by: saud-oai <saud@openai.com>

codex-rs/protocol/src/config_types.rs

@@ -166,8 +166,8 @@ pub enum ApprovalsReviewer {
     #[default]
     #[serde(rename = "user")]
     User,
-    #[serde(rename = "guardian_subagent", alias = "auto_review")]
-    #[strum(serialize = "guardian_subagent")]
+    #[serde(rename = "auto_review", alias = "guardian_subagent")]
+    #[strum(serialize = "auto_review")]
     AutoReview,
 }
 
@@ -760,7 +760,7 @@ mod tests {
         );
         assert_eq!(
             serde_json::to_string(&ApprovalsReviewer::AutoReview).expect("serialize reviewer"),
-            "\"guardian_subagent\""
+            "\"auto_review\""
         );
 
         for value in ["user", "auto_review", "guardian_subagent"] {