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/git_ui/src/branch_picker.rs

@@ -454,7 +454,7 @@ impl PickerDelegate for BranchListDelegate {
         _window: &mut Window,
         cx: &mut Context<Picker<Self>>,
     ) -> Option<Self::ListItem> {
-        let entry = &self.matches[ix];
+        let entry = &self.matches.get(ix)?;
 
         let (commit_time, author_name, subject) = entry
             .branch

crates/git_ui/src/picker_prompt.rs

@@ -216,7 +216,7 @@ impl PickerDelegate for PickerPromptDelegate {
         _window: &mut Window,
         _cx: &mut Context<Picker<Self>>,
     ) -> Option<Self::ListItem> {
-        let hit = &self.matches[ix];
+        let hit = &self.matches.get(ix)?;
         let shortened_option = util::truncate_and_trailoff(&hit.string, self.max_match_length);
 
         Some(