fix(tui): implement models:fetch-and-pick async handler

The /models command (and aliases /lm, /list-models) was dispatching to
'models:fetch-and-pick' but the async handler was never implemented.

This fix adds the missing case in handle_async_command() that opens the
ModelPicker modal, fetching models from provider_manager.

Fixes: missing handler causing 'Command not yet implemented' message

Author: echobt

src/cortex-tui/src/runner/event_loop/commands.rs

@@ -390,6 +390,9 @@ impl EventLoop {
             "history" => {
                 self.handle_history();
             }
+            "models:fetch-and-pick" => {
+                self.handle_open_modal(ModalType::ModelPicker).await;
+            }
             _ => {
                 self.app_state
                     .toasts