Preserve null developer instructions (#16976)

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

codex-rs/core/tests/suite/personality_migration.rs

@@ -71,6 +71,7 @@ async fn write_rollout_with_user_event(dir: &Path, thread_id: ThreadId) -> io::R
             agent_role: None,
             model_provider: None,
             base_instructions: None,
+            developer_instructions: None,
             dynamic_tools: None,
             memory_mode: None,
         },
@@ -116,6 +117,7 @@ async fn write_rollout_with_meta_only(dir: &Path, thread_id: ThreadId) -> io::Re
             agent_role: None,
             model_provider: None,
             base_instructions: None,
+            developer_instructions: None,
             dynamic_tools: None,
             memory_mode: None,
         },

codex-rs/core/tests/suite/rollout_list_find.rs

@@ -174,6 +174,7 @@ async fn find_locates_rollout_file_written_by_recorder() -> std::io::Result<()>
             /*forked_from_id*/ None,
             SessionSource::Exec,
             Some(BaseInstructions::default()),
+            /*developer_instructions*/ None,
             Vec::new(),
             EventPersistenceMode::Limited,
         ),

codex-rs/core/tests/suite/sqlite_state.rs

@@ -146,6 +146,7 @@ async fn backfill_scans_existing_rollouts() -> Result<()> {
                     agent_role: None,
                     model_provider: None,
                     base_instructions: None,
+                    developer_instructions: None,
                     dynamic_tools: Some(dynamic_tools_for_hook),
                     memory_mode: None,
                 },