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

crates/assistant/src/context.rs

@@ -1999,6 +1999,11 @@ impl Context {
                                     });
 
                                 match event {
+                                    LanguageModelCompletionEvent::Stop(reason) => match reason {
+                                        language_model::StopReason::ToolUse => {}
+                                        language_model::StopReason::EndTurn => {}
+                                        language_model::StopReason::MaxTokens => {}
+                                    },
                                     LanguageModelCompletionEvent::Text(chunk) => {
                                         buffer.edit(
                                             [(