### What happened?

The v3 `parallel_request_limiter` (`PROXY_HOOKS["parallel_request_limiter"]`, default since the v3 rewrite) populates `x-ratelimit-{descriptor}-{remaining,limit}-{requests,tokens}` headers from `async_post_call_success_hook` by mutating `response._hidden_params["additional_headers"]`. Two response paths silently drop these headers:

1. **Streaming responses** — for `stream=true` requests the SSE response headers are flushed to the client *before* `async_post_call_success_hook` runs. The hook still mutates `_hidden_params`, but the client never sees the resulting `x-ratelimit-*` keys. Affected: `/v1/chat/completions` stream=true, `/v1/messages` stream=true, `/v1/responses` stream=true. 2. **Plain-dict responses** — when `response` is a plain `dict` with no `_hidden_params` attribute the hook short-circuits at `if hasattr(response, "_hidden_params")` (`parallel_request_limiter_v3.py:2751`-2754 on `main`). Affected: `/v1/messages` non-streaming.

Both paths still increment the rate-limit counters correctly — only the *visibility* via headers is broken, which makes client-side quota tracking unreliable for any caller using the affected endpoints.

The hook is the on…