Add exec output-schema parameter (#4079)

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

codex-rs/mcp-server/src/codex_message_processor.rs

@@ -1009,6 +1009,7 @@ impl CodexMessageProcessor {
                 model,
                 effort,
                 summary,
+                final_output_json_schema: None,
             })
             .await;