Load models from static file (#8153)

- Load models from static file as a fallback
- Make API users use this file directly
- Add tests to make sure updates to the file always serialize

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

@@ -762,7 +762,7 @@ async fn includes_configured_effort_in_request() -> anyhow::Result<()> {
 }
 
 #[tokio::test(flavor = "multi_thread", worker_threads = 2)]
-async fn includes_no_effort_in_request() -> anyhow::Result<()> {
+async fn includes_default_effort_in_request() -> anyhow::Result<()> {
     skip_if_no_network!(Ok(()));
     let server = MockServer::start().await;
 
@@ -791,7 +791,7 @@ async fn includes_no_effort_in_request() -> anyhow::Result<()> {
             .get("reasoning")
             .and_then(|t| t.get("effort"))
             .and_then(|v| v.as_str()),
-        None
+        Some("medium")
     );
 
     Ok(())