diagnostics: Fix losing focus when activating from diagnostics view (#25517)

Closes #25509

Changes:
- If active item is already diagnostics, don't try to focus it again.

Instead of not focusing, should it just not activate instead? Something
like:

            if !workspace
                .active_item(cx)
                .map(|item| item.item_id() == existing.item_id())
                .unwrap_or(false)
            {
workspace.activate_item(&existing, true, true, window, cx);
            }


Release Notes:

- N/A

Author: 5brian

crates/diagnostics/src/diagnostics.rs

@@ -311,7 +311,10 @@ impl ProjectDiagnosticsEditor {
         cx: &mut Context<Workspace>,
     ) {
         if let Some(existing) = workspace.item_of_type::<ProjectDiagnosticsEditor>(cx) {
-            workspace.activate_item(&existing, true, true, window, cx);
+            let is_active = workspace
+                .active_item(cx)
+                .is_some_and(|item| item.item_id() == existing.item_id());
+            workspace.activate_item(&existing, true, !is_active, window, cx);
         } else {
             let workspace_handle = cx.entity().downgrade();