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/toolchain_selector/src/toolchain_selector.rs

@@ -1011,8 +1011,8 @@ impl PickerDelegate for ToolchainSelectorDelegate {
         _: &mut Window,
         cx: &mut Context<Picker<Self>>,
     ) -> Option<Self::ListItem> {
-        let mat = &self.matches[ix];
-        let (toolchain, scope) = &self.candidates[mat.candidate_id];
+        let mat = &self.matches.get(ix)?;
+        let (toolchain, scope) = &self.candidates.get(mat.candidate_id)?;
 
         let label = toolchain.name.clone();
         let path = Self::relativize_path(toolchain.path.clone(), &self.worktree_abs_path_root);