[codex] Propagate rate limit reached type (#18227)

## Summary

First PR in the split from #17956.

- adds the core/app-server `RateLimitReachedType` shape
- maps backend `rate_limit_reached_type` into Codex rate-limit snapshots
- carries the field through app-server notifications/responses and
generated schemas
- updates existing constructors/tests for the new optional field

## Validation

- `cargo test -p codex-backend-client`
- `cargo test -p codex-app-server-protocol`
- `cargo test -p codex-app-server rate_limits`
- `cargo test -p codex-tui workspace_`
- `cargo test -p codex-tui status_`
- `just fmt`
- `just fix -p codex-backend-client`
- `just fix -p codex-app-server-protocol`
- `just fix -p codex-app-server`
- `just fix -p codex-tui`

codex-rs/core/tests/suite/client_websockets.rs

@@ -1033,7 +1033,8 @@ async fn responses_websocket_usage_limit_error_emits_rate_limit_event() {
                     "resets_at": null
                 },
                 "credits": null,
-                "plan_type": null
+                "plan_type": null,
+                "rate_limit_reached_type": null
             }
         })
     );