Preserve null developer instructions (#16976)

Preserve explicit null developer-instruction overrides across app-server
resume and fork flows.

codex-rs/rollout/src/recorder_tests.rs

@@ -74,6 +74,7 @@ async fn recorder_materializes_only_after_explicit_persist() -> std::io::Result<
             /*forked_from_id*/ None,
             SessionSource::Exec,
             Some(BaseInstructions::default()),
+            /*developer_instructions*/ None,
             Vec::new(),
             EventPersistenceMode::Limited,
         ),
@@ -167,6 +168,7 @@ async fn metadata_irrelevant_events_touch_state_db_updated_at() -> std::io::Resu
             /*forked_from_id*/ None,
             SessionSource::Cli,
             Some(BaseInstructions::default()),
+            /*developer_instructions*/ None,
             Vec::new(),
             EventPersistenceMode::Limited,
         ),