agent: Add thread history import telemetry (#59050)

## Summary

- Emit `Thread History Viewed` from the shared archive/history opening
path so it covers button, keyboard, and onboarding entry points.
- Emit `Agent Threads Import Clicked` when users open external-agent
thread import from thread history or onboarding.
- Emit `Agent Threads Import Clicked` when users click cross-channel
thread import onboarding.

## Tests

- `cargo fmt --check`
- `git diff --check`
- `./script/clippy -p sidebar`
- `cargo test -p sidebar`

Release Notes:

- N/A

crates/sidebar/src/sidebar.rs

@@ -7458,7 +7458,21 @@ impl Sidebar {
             .map(|w| w.read(cx).workspace().clone())
     }
 
-    fn show_thread_import_modal(&mut self, window: &mut Window, cx: &mut Context<Self>) {
+    fn show_thread_import_modal(
+        &mut self,
+        source: &'static str,
+        window: &mut Window,
+        cx: &mut Context<Self>,
+    ) {
+        telemetry::event!(
+            "Agent Threads Import Clicked",
+            source = source,
+            side = match self.side(cx) {
+                SidebarSide::Left => "left",
+                SidebarSide::Right => "right",
+            }
+        );
+
         let Some(active_workspace) = self.active_workspace(cx) else {
             return;
         };
@@ -7514,7 +7528,7 @@ impl Sidebar {
     ) -> impl IntoElement {
         let on_import = cx.listener(|this, _, window, cx| {
             this.show_archive(window, cx);
-            this.show_thread_import_modal(window, cx);
+            this.show_thread_import_modal("external_agent_onboarding", window, cx);
         });
         render_import_onboarding_banner(
             "acp",
@@ -7553,6 +7567,14 @@ impl Sidebar {
         );
 
         let on_import = cx.listener(|this, _, _window, cx| {
+            telemetry::event!(
+                "Agent Threads Import Clicked",
+                source = "cross_channel_onboarding",
+                side = match this.side(cx) {
+                    SidebarSide::Left => "left",
+                    SidebarSide::Right => "right",
+                }
+            );
             CrossChannelImportOnboarding::dismiss(cx);
             if let Some(workspace) = this.active_workspace(cx) {
                 workspace.update(cx, |workspace, cx| {
@@ -7583,11 +7605,6 @@ impl Sidebar {
     ) {
         match &self.view {
             SidebarView::ThreadList => {
-                let side = match self.side(cx) {
-                    SidebarSide::Left => "left",
-                    SidebarSide::Right => "right",
-                };
-                telemetry::event!("Thread History Viewed", side = side);
                 self.show_archive(window, cx);
             }
             SidebarView::Archive(_) => self.show_thread_list(window, cx),
@@ -7595,6 +7612,12 @@ impl Sidebar {
     }
 
     fn show_archive(&mut self, window: &mut Window, cx: &mut Context<Self>) {
+        let side = match self.side(cx) {
+            SidebarSide::Left => "left",
+            SidebarSide::Right => "right",
+        };
+        telemetry::event!("Thread History Viewed", side = side);
+
         let Some(active_workspace) = self
             .multi_workspace
             .upgrade()
@@ -7639,7 +7662,7 @@ impl Sidebar {
                     this.restoring_tasks.remove(thread_id);
                 }
                 ThreadsArchiveViewEvent::Import => {
-                    this.show_thread_import_modal(window, cx);
+                    this.show_thread_import_modal("thread_history", window, cx);
                 }
             },
         );