Add exec output-schema parameter (#4079)

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

codex-rs/core/tests/suite/mod.rs

@@ -7,6 +7,7 @@ mod compact_resume_fork;
 mod exec;
 mod exec_stream_events;
 mod fork_conversation;
+mod json_result;
 mod live_cli;
 mod model_overrides;
 mod prompt_caching;