[app-server] read rate limits API (#5302)

Adds a `GET account/rateLimits/read` API to app-server. This calls the
codex backend to fetch the user's current rate limits.

This would be helpful in checking rate limits without having to send a
message.

For calling the codex backend usage API, I generated the types and
manually copied the relevant ones into `codex-backend-openapi-types`.
It'll be nice to extend our internal openapi generator to support Rust
so we don't have to run these manual steps.

# External (non-OpenAI) Pull Request Requirements

Before opening this Pull Request, please read the dedicated
"Contributing" markdown file or your PR may be closed:
https://github.com/openai/codex/blob/main/docs/contributing.md

If your PR conforms to our contribution guidelines, replace this text
with a detailed and high quality description of your changes.

codex-rs/codex-backend-openapi-models/src/models/rate_limit_window_snapshot.rs

@@ -0,0 +1,40 @@
+/*
+ * codex-backend
+ *
+ * codex-backend
+ *
+ * The version of the OpenAPI document: 0.0.1
+ *
+ * Generated by: https://openapi-generator.tech
+ */
+
+use serde::Deserialize;
+use serde::Serialize;
+
+#[derive(Clone, Default, Debug, PartialEq, Serialize, Deserialize)]
+pub struct RateLimitWindowSnapshot {
+    #[serde(rename = "used_percent")]
+    pub used_percent: i32,
+    #[serde(rename = "limit_window_seconds")]
+    pub limit_window_seconds: i32,
+    #[serde(rename = "reset_after_seconds")]
+    pub reset_after_seconds: i32,
+    #[serde(rename = "reset_at")]
+    pub reset_at: i32,
+}
+
+impl RateLimitWindowSnapshot {
+    pub fn new(
+        used_percent: i32,
+        limit_window_seconds: i32,
+        reset_after_seconds: i32,
+        reset_at: i32,
+    ) -> RateLimitWindowSnapshot {
+        RateLimitWindowSnapshot {
+            used_percent,
+            limit_window_seconds,
+            reset_after_seconds,
+            reset_at,
+        }
+    }
+}