Agent chat: queue messages while the agent works, Send now, Stop, New chat, and a model picker (#1566, #1571) (#1566)

Thanks @bherbruck.

crates/ui-egui/src/agent_chat/backend.rs

@@ -40,6 +40,8 @@ pub enum Event {
     AuthStatus(String),
     /// Its own modes (an ACP agent's `modes`: ask, accept edits, plan, …) and the current one.
     Modes { current: String, available: Vec<(String, String)> },
+    /// Its models (empty `available`: only the current one changed).
+    Models { current: String, available: Vec<super::acp::ModelInfo> },
     /// The message is answered (`reason`: why it stopped, empty for a normal end).
     TurnEnded { reason: String },
     /// Something failed; the backend is still running unless [`Event::Exited`] follows.
@@ -69,10 +71,15 @@ pub trait ChatBackend {
     fn prompt(&mut self, parts: &[PromptPart]);
     /// Stop the message being answered; open permission questions count as cancelled.
     fn cancel(&mut self);
+    /// Start a fresh conversation on the running backend (no restart); [`Event::Ready`] follows.
+    /// A message being answered is cancelled first.
+    fn new_session(&mut self);
     /// Answer an [`Event::Permission`] with one of its options (`None`: cancelled).
     fn answer_permission(&mut self, key: u64, option: Option<&str>);
     /// Ask the backend to sign in by itself with one of its methods (it opens the browser).
     fn authenticate(&mut self, method: &str);
+    /// Use one of the models it reported in [`Event::Models`].
+    fn set_model(&mut self, _model: &str) {}
     /// Switch to one of the modes it reported in [`Event::Modes`].
     fn set_mode(&mut self, _mode: &str) {}
     /// Its process, for status and tests.