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/tests/suite/client_websockets.rs

@@ -1654,9 +1654,9 @@ fn prompt_with_input(input: Vec<ResponseItem>) -> Prompt {
 
 fn prompt_with_input_and_instructions(input: Vec<ResponseItem>, instructions: &str) -> Prompt {
     let mut prompt = prompt_with_input(input);
-    prompt.base_instructions = BaseInstructions {
+    prompt.base_instructions = Some(BaseInstructions {
         text: instructions.to_string(),
-    };
+    });
     prompt
 }