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() {