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/agent2/src/tools/terminal_tool.rs

@@ -319,7 +319,7 @@ mod tests {
     use theme::ThemeSettings;
     use util::test::TempTree;
 
-    use crate::AgentResponseEvent;
+    use crate::ThreadEvent;
 
     use super::*;
 
@@ -396,7 +396,7 @@ mod tests {
             });
             cx.run_until_parked();
             let event = stream_rx.try_next();
-            if let Ok(Some(Ok(AgentResponseEvent::ToolCallAuthorization(auth)))) = event {
+            if let Ok(Some(Ok(ThreadEvent::ToolCallAuthorization(auth)))) = event {
                 auth.response.send(auth.options[0].id.clone()).unwrap();
             }