agent2: Remove model param from Thread::send method (#35936)

It instead uses the currently selected model

Release Notes:

- N/A

crates/agent2/src/thread.rs

@@ -200,11 +200,11 @@ impl Thread {
     /// The returned channel will report all the occurrences in which the model stops before erroring or ending its turn.
     pub fn send(
         &mut self,
-        model: Arc<dyn LanguageModel>,
         content: impl Into<MessageContent>,
         cx: &mut Context<Self>,
     ) -> mpsc::UnboundedReceiver<Result<AgentResponseEvent, LanguageModelCompletionError>> {
         let content = content.into();
+        let model = self.selected_model.clone();
         log::info!("Thread::send called with model: {:?}", model.name());
         log::debug!("Thread::send content: {:?}", content);