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/core/src/config_loader/tests.rs

@@ -905,7 +905,7 @@ model_instructions_file = "child.txt"
         .await?;
 
     assert_eq!(
-        config.base_instructions.as_deref(),
+        config.base_instructions.as_ref().and_then(Option::as_deref),
         Some("child instructions")
     );
 
@@ -941,7 +941,7 @@ async fn cli_override_model_instructions_file_sets_base_instructions() -> std::i
         .await?;
 
     assert_eq!(
-        config.base_instructions.as_deref(),
+        config.base_instructions.as_ref().and_then(Option::as_deref),
         Some("cli override instructions")
     );