Route approvals through the extension decision API (#43432)

## What changed

- Let approval extensions choose between cached approval, synchronous review, and a user prompt for tool and permission requests, while core enforces mandatory Guardian and fresh-review requirements.
- Carry an optional `review_reason` on Guardian assessment events and use it to trigger strict-review notifications, retaining the fallback for older events.
- Require synchronous review when cached evidence has an unusable encrypted parent compaction.

## Testing

Add regression coverage for manual prompts and cached approvals with non-UTF-8 working directories. Update approval contributors in existing tests to use the decision API.

GitOrigin-RevId: a4e66416070bdaa881daa41429c49021629f44c1

codex-rs/app-server-protocol/src/protocol/item_builders_tests.rs

@@ -129,6 +129,7 @@ fn guardian_stdin_reviews_preserve_parent_command_history() {
     assert!(expected[1].items.is_empty());
 
     let assessment = GuardianAssessmentEvent {
+        review_reason: None,
         id: "review-stdin".into(),
         target_item_id: Some("terminal-command".into()),
         plugin_id: None,
@@ -159,6 +160,7 @@ fn guardian_stdin_reviews_preserve_parent_command_history() {
         ] {
             items.push(RolloutItem::EventMsg(EventMsg::GuardianAssessment(
                 GuardianAssessmentEvent {
+                    review_reason: None,
                     turn_id: turn_id.into(),
                     status,
                     ..assessment.clone()