[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

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"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_details.rs

@@ -0,0 +1,46 @@
+/*
+ * 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 RateLimitStatusDetails {
+    #[serde(rename = "allowed")]
+    pub allowed: bool,
+    #[serde(rename = "limit_reached")]
+    pub limit_reached: bool,
+    #[serde(
+        rename = "primary_window",
+        default,
+        with = "::serde_with::rust::double_option",
+        skip_serializing_if = "Option::is_none"
+    )]
+    pub primary_window: Option<Option<Box<models::RateLimitWindowSnapshot>>>,
+    #[serde(
+        rename = "secondary_window",
+        default,
+        with = "::serde_with::rust::double_option",
+        skip_serializing_if = "Option::is_none"
+    )]
+    pub secondary_window: Option<Option<Box<models::RateLimitWindowSnapshot>>>,
+}
+
+impl RateLimitStatusDetails {
+    pub fn new(allowed: bool, limit_reached: bool) -> RateLimitStatusDetails {
+        RateLimitStatusDetails {
+            allowed,
+            limit_reached,
+            primary_window: None,
+            secondary_window: None,
+        }
+    }
+}