assistant: Propagate LLM stop reason upwards (#17358)

This PR makes it so we propagate the `stop_reason` from Anthropic up to
the Assistant so that we can take action based on it.

The `extract_content_from_events` function was moved from `anthropic` to
the `anthropic` module in `language_model` since it is more useful if it
is able to name the `LanguageModelCompletionEvent` type, as otherwise
we'd need an additional layer of plumbing.

Release Notes:

- N/A