fix(llm): request stream usage by default with extra_body override (#822)

* fix(llm): request stream usage by default with extra_body override

Streaming requests currently receive token usage only when the provider
volunteers it: OpenAI-compatible servers omit usage from streams unless
stream_options.include_usage is set, so streamed reviews silently lose
token and cost accounting.

Set stream_options.include_usage=true by default when streaming. The
existing extra_body passthrough stays in control: an explicit
stream_options object replaces the default, an explicit null suppresses
the field entirely for gateways that reject stream_options, and
non-streaming requests never carry the field (mirroring the #647
treatment of the "stream" key).

* fix(llm): keep include_usage on when stream_options configures unrelated options

An explicit stream_options object in extra_body used to replace the
include_usage default wholesale, so configuring an unrelated stream
option silently disabled usage accounting. include_usage now stays
requested unless the config spells it out (or suppresses stream_options
with an explicit null).