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/protocol/src/protocol.rs

@@ -2173,7 +2173,7 @@ pub struct TurnStartedEvent {
     pub collaboration_mode_kind: ModeKind,
 }
 
-#[derive(Debug, Clone, Deserialize, Serialize, JsonSchema, TS)]
+#[derive(Debug, Clone, Deserialize, Serialize, PartialEq, JsonSchema, TS)]
 pub struct ThreadSettingsAppliedEvent {
     /// Logical task that owns this snapshot, independent of the physical rollout file.
     /// Absent in older histories; copied snapshots retain their original owner's ID.