agent: Refresh UI when context or thread history changes (#28188)

I found a few more cases where the UI wasn't updated immediately after
an interaction.

Release Notes:

- agent: Fixed delay after removing threads from "Past Interactions"
- agent: Fixed delay after adding/remove context via keyboard

crates/agent/src/context_picker/completion_provider.rs

@@ -232,8 +232,8 @@ impl ContextPickerCompletionProvider {
                                 url_to_fetch.to_string(),
                             ))
                             .await?;
-                        context_store.update(cx, |context_store, _| {
-                            context_store.add_fetched_url(url_to_fetch.to_string(), content)
+                        context_store.update(cx, |context_store, cx| {
+                            context_store.add_fetched_url(url_to_fetch.to_string(), content, cx)
                         })
                     })
                     .detach_and_log_err(cx);