[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/Cargo.lock

@@ -182,7 +182,10 @@ version = "0.0.0"
 dependencies = [
  "anyhow",
  "assert_cmd",
+ "base64",
+ "chrono",
  "codex-app-server-protocol",
+ "codex-core",
  "serde",
  "serde_json",
  "tokio",
@@ -834,8 +837,10 @@ dependencies = [
  "app_test_support",
  "assert_cmd",
  "base64",
+ "chrono",
  "codex-app-server-protocol",
  "codex-arg0",
+ "codex-backend-client",
  "codex-common",
  "codex-core",
  "codex-file-search",
@@ -917,6 +922,8 @@ version = "0.0.0"
 dependencies = [
  "anyhow",
  "codex-backend-openapi-models",
+ "codex-core",
+ "codex-protocol",
  "pretty_assertions",
  "reqwest",
  "serde",
@@ -929,6 +936,7 @@ version = "0.0.0"
 dependencies = [
  "serde",
  "serde_json",
+ "serde_with",
 ]
 
 [[package]]