[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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codex-rs/app-server/tests/common/mcp_process.rs

@@ -236,6 +236,11 @@ impl McpProcess {
         self.send_request("getUserAgent", None).await
     }
 
+    /// Send an `account/rateLimits/read` JSON-RPC request.
+    pub async fn send_get_account_rate_limits_request(&mut self) -> anyhow::Result<i64> {
+        self.send_request("account/rateLimits/read", None).await
+    }
+
     /// Send a `userInfo` JSON-RPC request.
     pub async fn send_user_info_request(&mut self) -> anyhow::Result<i64> {
         self.send_request("userInfo", None).await