[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/backend-client/src/client.rs

@@ -1,7 +1,13 @@
 use crate::types::CodeTaskDetailsResponse;
 use crate::types::PaginatedListTaskListItem;
+use crate::types::RateLimitStatusPayload;
+use crate::types::RateLimitWindowSnapshot;
 use crate::types::TurnAttemptsSiblingTurnsResponse;
 use anyhow::Result;
+use codex_core::auth::CodexAuth;
+use codex_core::default_client::get_codex_user_agent;
+use codex_protocol::protocol::RateLimitSnapshot;
+use codex_protocol::protocol::RateLimitWindow;
 use reqwest::header::AUTHORIZATION;
 use reqwest::header::CONTENT_TYPE;
 use reqwest::header::HeaderMap;
@@ -64,6 +70,17 @@ impl Client {
         })
     }
 
+    pub async fn from_auth(base_url: impl Into<String>, auth: &CodexAuth) -> Result<Self> {
+        let token = auth.get_token().await.map_err(anyhow::Error::from)?;
+        let mut client = Self::new(base_url)?
+            .with_user_agent(get_codex_user_agent())
+            .with_bearer_token(token);
+        if let Some(account_id) = auth.get_account_id() {
+            client = client.with_chatgpt_account_id(account_id);
+        }
+        Ok(client)
+    }
+
     pub fn with_bearer_token(mut self, token: impl Into<String>) -> Self {
         self.bearer_token = Some(token.into());
         self
@@ -138,6 +155,17 @@ impl Client {
         }
     }
 
+    pub async fn get_rate_limits(&self) -> Result<RateLimitSnapshot> {
+        let url = match self.path_style {
+            PathStyle::CodexApi => format!("{}/api/codex/usage", self.base_url),
+            PathStyle::ChatGptApi => format!("{}/wham/usage", self.base_url),
+        };
+        let req = self.http.get(&url).headers(self.headers());
+        let (body, ct) = self.exec_request(req, "GET", &url).await?;
+        let payload: RateLimitStatusPayload = self.decode_json(&url, &ct, &body)?;
+        Ok(Self::rate_limit_snapshot_from_payload(payload))
+    }
+
     pub async fn list_tasks(
         &self,
         limit: Option<i32>,
@@ -241,4 +269,49 @@ impl Client {
             Err(e) => anyhow::bail!("Decode error for {url}: {e}; content-type={ct}; body={body}"),
         }
     }
+
+    // rate limit helpers
+    fn rate_limit_snapshot_from_payload(payload: RateLimitStatusPayload) -> RateLimitSnapshot {
+        let Some(details) = payload
+            .rate_limit
+            .and_then(|inner| inner.map(|boxed| *boxed))
+        else {
+            return RateLimitSnapshot {
+                primary: None,
+                secondary: None,
+            };
+        };
+
+        RateLimitSnapshot {
+            primary: Self::map_rate_limit_window(details.primary_window),
+            secondary: Self::map_rate_limit_window(details.secondary_window),
+        }
+    }
+
+    fn map_rate_limit_window(
+        window: Option<Option<Box<RateLimitWindowSnapshot>>>,
+    ) -> Option<RateLimitWindow> {
+        let snapshot = match window {
+            Some(Some(snapshot)) => *snapshot,
+            _ => return None,
+        };
+
+        let used_percent = f64::from(snapshot.used_percent);
+        let window_minutes = Self::window_minutes_from_seconds(snapshot.limit_window_seconds);
+        let resets_at = Some(i64::from(snapshot.reset_at));
+        Some(RateLimitWindow {
+            used_percent,
+            window_minutes,
+            resets_at,
+        })
+    }
+
+    fn window_minutes_from_seconds(seconds: i32) -> Option<i64> {
+        if seconds <= 0 {
+            return None;
+        }
+
+        let seconds_i64 = i64::from(seconds);
+        Some((seconds_i64 + 59) / 60)
+    }
 }

codex-rs/backend-client/src/types.rs

@@ -1,4 +1,8 @@
 pub use codex_backend_openapi_models::models::PaginatedListTaskListItem;
+pub use codex_backend_openapi_models::models::PlanType;
+pub use codex_backend_openapi_models::models::RateLimitStatusDetails;
+pub use codex_backend_openapi_models::models::RateLimitStatusPayload;
+pub use codex_backend_openapi_models::models::RateLimitWindowSnapshot;
 pub use codex_backend_openapi_models::models::TaskListItem;
 
 use serde::Deserialize;