Introduce recent files ambient context for assistant (#11791)

<img width="1637" alt="image"
src="https://github.com/zed-industries/zed/assets/482957/5aaec657-3499-42c9-9528-c83728f2a7a1">

Release Notes:

- Added a new ambient context feature that allows showing the model up
to three buffers (along with their diagnostics) that the user interacted
with recently.

---------

Co-authored-by: Nathan Sobo <nathan@zed.dev>

crates/tab_switcher/src/tab_switcher.rs

@@ -189,8 +189,8 @@ impl TabSwitcherDelegate {
         let pane = pane.read(cx);
         let mut history_indices = HashMap::default();
         pane.activation_history().iter().rev().enumerate().for_each(
-            |(history_index, entity_id)| {
-                history_indices.insert(entity_id, history_index);
+            |(history_index, history_entry)| {
+                history_indices.insert(history_entry.entity_id, history_index);
             },
         );