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.rs

@@ -81,7 +81,7 @@ pub enum RolloutRecorderParams {
         conversation_id: ThreadId,
         forked_from_id: Option<ThreadId>,
         source: SessionSource,
-        base_instructions: BaseInstructions,
+        base_instructions: Option<BaseInstructions>,
         dynamic_tools: Vec<DynamicToolSpec>,
         event_persistence_mode: EventPersistenceMode,
     },
@@ -110,7 +110,7 @@ impl RolloutRecorderParams {
         conversation_id: ThreadId,
         forked_from_id: Option<ThreadId>,
         source: SessionSource,
-        base_instructions: BaseInstructions,
+        base_instructions: Option<BaseInstructions>,
         dynamic_tools: Vec<DynamicToolSpec>,
         event_persistence_mode: EventPersistenceMode,
     ) -> Self {

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,
         ),

codex-rs/rollout/src/tests.rs

@@ -977,7 +977,7 @@ async fn test_get_thread_contents() {
 }
 
 #[tokio::test]
-async fn test_base_instructions_missing_in_meta_defaults_to_null() {
+async fn test_base_instructions_missing_in_meta_stays_missing() {
     let temp = TempDir::new().unwrap();
     let home = temp.path();
 
@@ -1011,10 +1011,7 @@ async fn test_base_instructions_missing_in_meta_defaults_to_null() {
         .await
         .expect("session meta head");
     let first = head.first().expect("first head entry");
-    assert_eq!(
-        first.get("base_instructions"),
-        Some(&serde_json::Value::Null)
-    );
+    assert_eq!(first.get("base_instructions"), None);
 }
 
 #[tokio::test]