Lay the groundwork to support history in agent2 (#36483)

This pull request introduces title generation and history replaying. We
still need to wire up the rest of the history but this gets us very
close. I extracted a lot of this code from `agent2-history` because that
branch was starting to get long-lived and there were lots of changes
since we started.

Release Notes:

- N/A

crates/acp_thread/src/connection.rs

@@ -5,11 +5,12 @@ use collections::IndexMap;
 use gpui::{Entity, SharedString, Task};
 use language_model::LanguageModelProviderId;
 use project::Project;
+use serde::{Deserialize, Serialize};
 use std::{any::Any, error::Error, fmt, path::Path, rc::Rc, sync::Arc};
 use ui::{App, IconName};
 use uuid::Uuid;
 
-#[derive(Clone, Debug, Eq, PartialEq)]
+#[derive(Debug, Clone, PartialEq, Eq, Serialize, Deserialize)]
 pub struct UserMessageId(Arc<str>);
 
 impl UserMessageId {
@@ -208,6 +209,7 @@ impl AgentModelList {
 mod test_support {
     use std::sync::Arc;
 
+    use action_log::ActionLog;
     use collections::HashMap;
     use futures::{channel::oneshot, future::try_join_all};
     use gpui::{AppContext as _, WeakEntity};
@@ -295,8 +297,16 @@ mod test_support {
             cx: &mut gpui::App,
         ) -> Task<gpui::Result<Entity<AcpThread>>> {
             let session_id = acp::SessionId(self.sessions.lock().len().to_string().into());
-            let thread =
-                cx.new(|cx| AcpThread::new("Test", self.clone(), project, session_id.clone(), cx));
+            let action_log = cx.new(|_| ActionLog::new(project.clone()));
+            let thread = cx.new(|_cx| {
+                AcpThread::new(
+                    "Test",
+                    self.clone(),
+                    project,
+                    action_log,
+                    session_id.clone(),
+                )
+            });
             self.sessions.lock().insert(
                 session_id,
                 Session {