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/protocol/src/lib.rs

@@ -23,4 +23,5 @@ pub mod plan_tool;
 pub mod protocol;
 pub mod request_permissions;
 pub mod request_user_input;
+mod serde_helpers;
 pub mod user_input;