[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.

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

@@ -3,6 +3,7 @@
 // Currently export only the types referenced by the workspace
 // The process for this will change
 
+// Cloud Tasks
 pub mod code_task_details_response;
 pub use self::code_task_details_response::CodeTaskDetailsResponse;
 
@@ -20,3 +21,14 @@ pub use self::task_list_item::TaskListItem;
 
 pub mod paginated_list_task_list_item_;
 pub use self::paginated_list_task_list_item_::PaginatedListTaskListItem;
+
+// Rate Limits
+pub mod rate_limit_status_payload;
+pub use self::rate_limit_status_payload::PlanType;
+pub use self::rate_limit_status_payload::RateLimitStatusPayload;
+
+pub mod rate_limit_status_details;
+pub use self::rate_limit_status_details::RateLimitStatusDetails;
+
+pub mod rate_limit_window_snapshot;
+pub use self::rate_limit_window_snapshot::RateLimitWindowSnapshot;