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/analytics/src/events.rs

@@ -400,6 +400,7 @@ pub enum GuardianReviewedAction {
 
 #[derive(Clone, Serialize)]
 pub struct GuardianReviewEventParams {
+    pub guardian_context_mode: Option<&'static str>,
     pub thread_id: String,
     pub turn_id: String,
     pub review_id: String,
@@ -479,6 +480,7 @@ impl GuardianReviewTrackContext {
         completed_at_ms: u64,
     ) -> GuardianReviewEventParams {
         GuardianReviewEventParams {
+            guardian_context_mode: result.guardian_context_mode,
             thread_id: self.thread_id.clone(),
             turn_id: self.turn_id.clone(),
             review_id: self.review_id.clone(),
@@ -531,6 +533,7 @@ impl GuardianReviewTrackContext {
 
 #[derive(Debug)]
 pub struct GuardianReviewAnalyticsResult {
+    pub guardian_context_mode: Option<&'static str>,
     pub decision: GuardianReviewDecision,
     pub terminal_status: GuardianReviewTerminalStatus,
     pub failure_reason: Option<GuardianReviewFailureReason>,
@@ -556,6 +559,7 @@ pub struct GuardianReviewAnalyticsResult {
 impl GuardianReviewAnalyticsResult {
     pub fn without_session() -> Self {
         Self {
+            guardian_context_mode: None,
             decision: GuardianReviewDecision::Denied,
             terminal_status: GuardianReviewTerminalStatus::FailedClosed,
             failure_reason: None,

codex-rs/analytics/src/guardian_v2.rs

@@ -21,6 +21,7 @@ pub struct GuardianV2Event {
 #[serde(untagged)]
 pub enum GuardianV2EventKind {
     Classification {
+        guardian_context_mode: &'static str,
         outcome: &'static str,
         risk_level: Option<&'static str>,
         duration_ms: u64,

codex-rs/analytics/src/tests/suite/approvals_tests.rs

@@ -292,6 +292,7 @@ async fn guardian_review_event_ingests_custom_fact_with_optional_target_item() {
         .ingest(
             AnalyticsFact::Custom(CustomAnalyticsFact::GuardianReview(Box::new(
                 GuardianReviewEventParams {
+                    guardian_context_mode: None,
                     thread_id: "thread-guardian".to_string(),
                     turn_id: "turn-guardian".to_string(),
                     review_id: "review-guardian".to_string(),