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.

Cargo.lock

@@ -285,7 +285,7 @@ checksum = "1d07550c9036bf2ae0c684c4297d503f838287c83c53686d05370d0e139ae570"
 
 [[package]]
 name = "cortiq-gateway"
-version = "0.2.48"
+version = "0.2.49"
 dependencies = [
  "anyhow",
  "arc-swap",