Preserve null developer instructions (#16976)

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

codex-rs/rollout/src/tests.rs

@@ -1140,6 +1140,7 @@ async fn test_updated_at_uses_file_mtime() -> Result<()> {
                 agent_role: None,
                 model_provider: Some("test-provider".into()),
                 base_instructions: None,
+                developer_instructions: None,
                 dynamic_tools: None,
                 memory_mode: None,
             },