fix(stream): no choice-less chunks unless the client asks for usage

Every stream used to carry two chunks with an empty choices array: usage,
which the upstream emits BEFORE the stop reason, and a cortiq_cost_usd chunk
the gateway added itself. OpenAI ships the usage chunk only when the request
sets stream_options: {"include_usage": true}, and chat UIs do not set it -
which is why the same tool, in the same Open WebUI, sees neither chunk from
Groq and two from us.

Honour the flag. With no stream_options a stream is now exactly
role -> content... -> tool_calls -> finish_reason -> [DONE], with no
choice-less chunks at all. When usage is requested it ships once, after the
stop reason, with cortiq_cost_usd folded into the same chunk rather than
trailing behind it.

Token accounting, cost and the X-Cortiq-* headers are untouched: those numbers
were never read back off the wire.

Verified: the default stream matches the reference shape byte for byte, the
opt-in stream matches OpenAI's, the official OpenAI SDK reassembles the
streamed tool call, and the full Open WebUI 0.11.0 tool loop still runs the
tool and answers from its result.

docs/PROTOCOLS.md

@@ -121,6 +121,12 @@ opens with a `role` delta, carries exactly one terminal `finish_reason`, and end
 failure arrives as an SSE `error` event. A client can therefore never be left waiting on a
 half-formed stream.
 
+Streaming carries no choice-less chunks unless asked: `usage` is emitted only when the
+request sets `stream_options: {"include_usage": true}`, and then exactly once, after the
+`finish_reason` chunk, with `cortiq_cost_usd` folded in. Without it a stream is
+`role → content… → tool_calls → finish_reason → [DONE]` and nothing else — the shape the
+hosted providers send.
+
 ### Tool calls
 
 `tools` and `tool_calls` are proxied through, with the wire kept clean:

docs/PROTOCOLS.ru.md

@@ -119,6 +119,12 @@ HTTP 400 — но не «голый» 422, который OpenAI-клиенты
 `data: [DONE]` — даже если upstream оборвался или упал на середине (тогда ошибка приходит
 SSE-событием `error`). Клиент никогда не остаётся ждать недосказанный поток.
 
+В стриме нет чанков без `choices`, если их не просили: `usage` отдаётся, только когда
+запрос содержит `stream_options: {"include_usage": true}`, и тогда ровно один раз — после
+чанка с `finish_reason`, с вложенным `cortiq_cost_usd`. Без него поток выглядит как
+`role → content… → tool_calls → finish_reason → [DONE]` и ничего больше — ровно так же,
+как у хостовых провайдеров.
+
 ### Tool calls
 
 `tools` и `tool_calls` проксируются, но провод держится чистым: