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/rollout/src/lib.rs

@@ -27,6 +27,7 @@ pub mod state_db;
 mod writer_lock;
 
 pub use codex_history::CompactedItem;
+pub use codex_history::CompactionResumeMetadata;
 pub use codex_history::InitialHistory;
 pub use codex_history::ResponseItemEnvelope;
 pub use codex_history::ResumedHistory;

codex-rs/rollout/src/metadata_tests.rs

@@ -328,6 +328,7 @@ fn builder_from_items_falls_back_to_filename() {
         window_id: None,
         compaction_response_id: None,
         latest_token_usage_record: None,
+        resume_metadata: None,
     })];
 
     let builder = builder_from_items(items.as_slice(), path.as_path()).expect("builder");