Honor null thread instructions (#16964)

- Treat explicit null thread instructions as a blank-slate override
while preserving omitted-field fallback behavior.
- Preserve null through rollout resume/fork and keep explicit empty
strings distinct.
- Add app-server v2 start/fork coverage for the tri-state instruction
params.

codex-rs/rollout/src/recorder_tests.rs

@@ -73,7 +73,7 @@ async fn recorder_materializes_only_after_explicit_persist() -> std::io::Result<
             thread_id,
             /*forked_from_id*/ None,
             SessionSource::Exec,
-            BaseInstructions::default(),
+            Some(BaseInstructions::default()),
             Vec::new(),
             EventPersistenceMode::Limited,
         ),
@@ -166,7 +166,7 @@ async fn metadata_irrelevant_events_touch_state_db_updated_at() -> std::io::Resu
             thread_id,
             /*forked_from_id*/ None,
             SessionSource::Cli,
-            BaseInstructions::default(),
+            Some(BaseInstructions::default()),
             Vec::new(),
             EventPersistenceMode::Limited,
         ),