[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_status_payload.rs

@@ -0,0 +1,65 @@
+/*
+ * codex-backend
+ *
+ * codex-backend
+ *
+ * The version of the OpenAPI document: 0.0.1
+ *
+ * Generated by: https://openapi-generator.tech
+ */
+
+use crate::models;
+use serde::Deserialize;
+use serde::Serialize;
+
+#[derive(Clone, Default, Debug, PartialEq, Serialize, Deserialize)]
+pub struct RateLimitStatusPayload {
+    #[serde(rename = "plan_type")]
+    pub plan_type: PlanType,
+    #[serde(
+        rename = "rate_limit",
+        default,
+        with = "::serde_with::rust::double_option",
+        skip_serializing_if = "Option::is_none"
+    )]
+    pub rate_limit: Option<Option<Box<models::RateLimitStatusDetails>>>,
+}
+
+impl RateLimitStatusPayload {
+    pub fn new(plan_type: PlanType) -> RateLimitStatusPayload {
+        RateLimitStatusPayload {
+            plan_type,
+            rate_limit: None,
+        }
+    }
+}
+
+#[derive(Clone, Copy, Debug, Eq, PartialEq, Ord, PartialOrd, Hash, Serialize, Deserialize)]
+pub enum PlanType {
+    #[serde(rename = "free")]
+    Free,
+    #[serde(rename = "go")]
+    Go,
+    #[serde(rename = "plus")]
+    Plus,
+    #[serde(rename = "pro")]
+    Pro,
+    #[serde(rename = "team")]
+    Team,
+    #[serde(rename = "business")]
+    Business,
+    #[serde(rename = "education")]
+    Education,
+    #[serde(rename = "quorum")]
+    Quorum,
+    #[serde(rename = "enterprise")]
+    Enterprise,
+    #[serde(rename = "edu")]
+    Edu,
+}
+
+impl Default for PlanType {
+    fn default() -> PlanType {
+        Self::Free
+    }
+}