samples: workflow_streams: lock llm-stream dependency group

The llm-stream dependency group was introduced in pyproject.toml
without a corresponding uv.lock update, so `uv sync --frozen --group
llm-stream` would fail or force a relock before scenario 5 could
run. Add the two missing entries (the package-optional-dependencies
list and the package-metadata requires-dev list) so frozen installs
work against the committed lock.

Found in a Codex review of the day's workflow_streams changes.

Author: jssmith