Add exec output-schema parameter (#4079)

Adds structured output to `exec` via the `--structured-output`
parameter.

codex-rs/core/src/codex/compact.rs

@@ -106,6 +106,7 @@ async fn run_compact_task_inner(
         input: turn_input,
         tools: Vec::new(),
         base_instructions_override: instructions_override,
+        output_schema: None,
     };
 
     let max_retries = turn_context.client.get_provider().stream_max_retries();