Add exec output-schema parameter (#4079)

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

codex-rs/core/src/chat_completions.rs

@@ -35,6 +35,12 @@ pub(crate) async fn stream_chat_completions(
     client: &reqwest::Client,
     provider: &ModelProviderInfo,
 ) -> Result<ResponseStream> {
+    if prompt.output_schema.is_some() {
+        return Err(CodexErr::UnsupportedOperation(
+            "output_schema is not supported for Chat Completions API".to_string(),
+        ));
+    }
+
     // Build messages array
     let mut messages = Vec::<serde_json::Value>::new();