Record Guardian context mode in review and classification telemetry (#49294)

## Why

Guardian review and classification telemetry lacks the context mode needed to distinguish `legacy`, `independent`, and `thread_owned` reviews.

## What changed

- Add `guardian_context_mode` to review and classification analytics and `context_mode` to their metrics.
- Capture the mode from the conversation history snapshot before review preparation or classification setup so failure metrics retain it, including when review preparation or reviewer startup fails.
- Use `unknown` for review metric tags when no context mode is available.

## Testing

Update metric assertions to cover `thread_owned` classification tags and the `unknown` review fallback.

GitOrigin-RevId: 79f7d11b3e4ccfc53681890d75fbc7c54e1ce7de

codex-rs/core/src/context/guardian_context_mode.rs

@@ -15,6 +15,15 @@ pub enum GuardianContextMode {
 }
 
 impl GuardianContextMode {
+    /// Stable, bounded label for the context actually selected for a review.
+    pub fn as_str(self) -> &'static str {
+        match self {
+            Self::Legacy => "legacy",
+            Self::Independent => "independent",
+            Self::ThreadOwned => "thread_owned",
+        }
+    }
+
     /// Read reviewer policy from the same snapshot as its evidence, including delayed reviews.
     pub fn from_history(history: &dyn ConversationHistorySnapshot) -> Self {
         if history.uses_independent_review_history() {

codex-rs/core/src/guardian/review_session_setup.rs

@@ -207,13 +207,22 @@ pub(crate) async fn run_guardian_review_session(
     pool: Arc<ReviewerPool<GuardianReviewSession>>,
     params: GuardianReviewSessionParams,
 ) -> (GuardianReviewSessionOutcome, GuardianReviewAnalyticsResult) {
-    match prepare_review(params).await {
+    let context_mode = GuardianContextMode::from_history(
+        params
+            .parent_history
+            .conversation_history_snapshot()
+            .as_ref(),
+    );
+    let (outcome, mut analytics) = match prepare_review(params).await {
         Ok(prepared) => pool.review(prepared).await,
         Err(error) => (
             GuardianReviewSessionOutcome::PromptBuildFailed(error),
             GuardianReviewAnalyticsResult::without_session(),
         ),
-    }
+    };
+    // Keep the captured mode even when preparation or reviewer startup fails.
+    analytics.guardian_context_mode = Some(context_mode.as_str());
+    (outcome, analytics)
 }
 
 pub(super) async fn prepare_review(