agent: Move ActiveThread and MessageEditor into ActiveView (#33462)

`ActiveThread` and `MessageEditor` only make sense when `active_view` is
`Thread`, so we moved them in there. This will make it easier to work on
new agent threads.

Release Notes:

- N/A

---------

Co-authored-by: Antonio Scandurra <me@as-cii.com>
Co-authored-by: Conrad Irwin <conrad.irwin@gmail.com>
Co-authored-by: Smit Barmase <heysmitbarmase@gmail.com>

crates/agent_ui/src/context_strip.rs

@@ -161,7 +161,7 @@ impl ContextStrip {
         let workspace = self.workspace.upgrade()?;
         let panel = workspace.read(cx).panel::<AgentPanel>(cx)?.read(cx);
 
-        if let Some(active_thread) = panel.active_thread() {
+        if let Some(active_thread) = panel.active_thread(cx) {
             let weak_active_thread = active_thread.downgrade();
 
             let active_thread = active_thread.read(cx);