Preserve null developer instructions (#16976)

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

codex-rs/app-server/tests/suite/v2/thread_resume.rs

@@ -377,6 +377,7 @@ stream_max_retries = 0
         agent_role: None,
         model_provider: Some("mock_provider".to_string()),
         base_instructions: None,
+        developer_instructions: None,
         dynamic_tools: None,
         memory_mode: None,
     };