Persist resume metadata in compaction checkpoints (#47323)

## Why

Compaction companion records do not represent all resume state, including the last started turn and previous turn settings. The selected multi-agent runtime also needs a persisted value when no companion turn context is present.

## What changed

- Add optional `resume_metadata` to `CompactedItem`, recording the multi-agent version, last started turn ID, and previous turn settings. Preserve the distinction between absent metadata and explicitly empty values while retaining compatibility with older records.
- Capture this metadata under the settings persistence lock alongside the compaction checkpoint.
- Rewrite inherited metadata to use the child agent's runtime, and clear previous turn settings when the child must rebuild context.
- Clear pending continuation state before delivering acceptance of standalone settings updates, while holding the persistence lock.

## Testing

Add coverage for metadata round trips with empty values, legacy reader compatibility, checkpoint companion records, inherited fork metadata, and continuation invalidation while settings event delivery is blocked.

GitOrigin-RevId: f30b7feaa7e2fdb5b65fc3c53b37fd7a7fe27903

codex-rs/thread-store/src/local/model_context_tests.rs

@@ -891,5 +891,6 @@ fn compacted(message: &str, replacement_history: Option<Vec<ResponseItem>>) -> R
         window_id: None,
         compaction_response_id: None,
         latest_token_usage_record: None,
+        resume_metadata: None,
     })
 }

codex-rs/thread-store/src/local/rollout_migration_tests.rs

@@ -248,6 +248,7 @@ fn compacted(replacement_history: Vec<ResponseItem>) -> RolloutItem {
         window_id: None,
         compaction_response_id: None,
         latest_token_usage_record: None,
+        resume_metadata: None,
     })
 }
 
@@ -1783,6 +1784,7 @@ async fn migration_compacts_subagent_prefix_and_does_not_project_it() {
                 window_id: None,
                 compaction_response_id: None,
                 latest_token_usage_record: None,
+                resume_metadata: None,
             }),
             RolloutItem::Compacted(CompactedItem {
                 message: "latest checkpoint".to_string(),
@@ -1807,6 +1809,7 @@ async fn migration_compacts_subagent_prefix_and_does_not_project_it() {
                 window_id: None,
                 compaction_response_id: None,
                 latest_token_usage_record: None,
+                resume_metadata: None,
             }),
             started("child-turn"),
             RolloutItem::TurnContext(TurnContextItem {

codex-rs/thread-store/src/thread_metadata_sync.rs

@@ -674,6 +674,7 @@ mod tests {
             window_id: None,
             compaction_response_id: None,
             latest_token_usage_record: None,
+            resume_metadata: None,
         });
 
         let first = sync