Preserve independent Guardian history across parent compaction (#48779)

## Why

Guardian reviews need to retain original evidence across parent compaction when checkpoint reuse is disabled, including after resume and rollback.

## What changed

- Make disabling `guardian_reuse_parent_compaction` select a bounded, independent review transcript while preserving thread-owned authorization.
- Keep synchronous reviews and asynchronous scoring independent of parent checkpoints, and allow reviewer sessions to continue using transcript deltas across parent compaction.
- Persist transcript entries with rollback provenance while remaining compatible with older checkpoint readers and metadata-free checkpoints.
- Exclude compaction output from the transcript and synthetic summaries from rollback turn boundaries. Use acceptance ordering to remove rolled-back evidence even when persistence order differs.

## Testing

Add regression coverage for reviewer continuity, evidence retention across compaction and resume, rollback after local and remote compaction, and checkpoint serialization compatibility.

GitOrigin-RevId: fea0852cc5537372854f4a34bacb6bfda6d13e88

codex-rs/ext/extension-api/src/capabilities/conversation_history.rs

@@ -36,6 +36,12 @@ pub trait ConversationHistorySnapshot: Send + Sync {
         self.retained_context().is_some()
     }
 
+    /// Whether the host retains a bounded review transcript independently of parent compaction.
+    /// Such snapshots must never seed reviewers with the parent's opaque checkpoint.
+    fn uses_independent_review_history(&self) -> bool {
+        false
+    }
+
     /// Latest opaque checkpoint, including unusable items, with its recorded producer.
     /// Hosts without provenance leave the producer unknown rather than using the live model.
     fn latest_compaction(&self) -> Option<CompactionCheckpoint<'_>> {