Fix crash when filtering items in Picker (#37929)

Closes #37617

We're already using `get` in a bunch of places, this PR updates the
remaining spots to follow the same pattern. Note that the `ix` we read
in `render_match` can sometimes be stale.

The likely reason is that we run the match-update logic asynchronously
(see
[here](https://github.com/zed-industries/zed/blob/138117e0b15664079f5526cb56168750382b49b9/crates/picker/src/picker.rs#L643)).
That means it's possible to render items after the list's [data
update](https://github.com/zed-industries/zed/blob/138117e0b15664079f5526cb56168750382b49b9/crates/picker/src/picker.rs#L652)
but before the [list
reset](https://github.com/zed-industries/zed/blob/138117e0b15664079f5526cb56168750382b49b9/crates/picker/src/picker.rs#L662),
in which case the `ix` can be greater than that of our updated data.

Release Notes:

- Fixed crash when filtering MCP tools.

crates/tab_switcher/src/tab_switcher.rs

@@ -649,10 +649,7 @@ impl PickerDelegate for TabSwitcherDelegate {
         window: &mut Window,
         cx: &mut Context<Picker<Self>>,
     ) -> Option<Self::ListItem> {
-        let tab_match = self
-            .matches
-            .get(ix)
-            .expect("Invalid matches state: no element for index {ix}");
+        let tab_match = self.matches.get(ix)?;
 
         let params = TabContentParams {
             detail: Some(tab_match.detail),