Preserve null developer instructions (#16976)

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

codex-rs/rollout/src/recorder.rs

@@ -82,6 +82,7 @@ pub enum RolloutRecorderParams {
         forked_from_id: Option<ThreadId>,
         source: SessionSource,
         base_instructions: Option<BaseInstructions>,
+        developer_instructions: Option<Option<String>>,
         dynamic_tools: Vec<DynamicToolSpec>,
         event_persistence_mode: EventPersistenceMode,
     },
@@ -111,6 +112,7 @@ impl RolloutRecorderParams {
         forked_from_id: Option<ThreadId>,
         source: SessionSource,
         base_instructions: Option<BaseInstructions>,
+        developer_instructions: Option<Option<String>>,
         dynamic_tools: Vec<DynamicToolSpec>,
         event_persistence_mode: EventPersistenceMode,
     ) -> Self {
@@ -119,6 +121,7 @@ impl RolloutRecorderParams {
             forked_from_id,
             source,
             base_instructions,
+            developer_instructions,
             dynamic_tools,
             event_persistence_mode,
         }
@@ -380,6 +383,7 @@ impl RolloutRecorder {
                     forked_from_id,
                     source,
                     base_instructions,
+                    developer_instructions,
                     dynamic_tools,
                     event_persistence_mode,
                 } => {
@@ -409,6 +413,7 @@ impl RolloutRecorder {
                         source,
                         model_provider: Some(config.model_provider_id().to_string()),
                         base_instructions: Some(base_instructions),
+                        developer_instructions,
                         dynamic_tools: if dynamic_tools.is_empty() {
                             None
                         } else {