[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/protocol/src/error_tests.rs

@@ -36,6 +36,7 @@ fn rate_limit_snapshot() -> RateLimitSnapshot {
         }),
         credits: None,
         plan_type: None,
+        rate_limit_reached_type: None,
     }
 }