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/agent_ui/src/agent_configuration/tool_picker.rs

@@ -318,7 +318,7 @@ impl PickerDelegate for ToolPickerDelegate {
         _window: &mut Window,
         cx: &mut Context<Picker<Self>>,
     ) -> Option<Self::ListItem> {
-        let item = &self.filtered_items[ix];
+        let item = &self.filtered_items.get(ix)?;
         match item {
             PickerItem::ContextServer { server_id, .. } => Some(
                 div()

crates/agent_ui/src/context_picker/file_context_picker.rs

@@ -160,7 +160,7 @@ impl PickerDelegate for FileContextPickerDelegate {
         _window: &mut Window,
         cx: &mut Context<Picker<Self>>,
     ) -> Option<Self::ListItem> {
-        let FileMatch { mat, .. } = &self.matches[ix];
+        let FileMatch { mat, .. } = &self.matches.get(ix)?;
 
         Some(
             ListItem::new(ix)

crates/agent_ui/src/context_picker/rules_context_picker.rs

@@ -146,7 +146,7 @@ impl PickerDelegate for RulesContextPickerDelegate {
         _window: &mut Window,
         cx: &mut Context<Picker<Self>>,
     ) -> Option<Self::ListItem> {
-        let thread = &self.matches[ix];
+        let thread = &self.matches.get(ix)?;
 
         Some(ListItem::new(ix).inset(true).toggle_state(selected).child(
             render_thread_context_entry(thread, self.context_store.clone(), cx),

crates/agent_ui/src/context_picker/symbol_context_picker.rs

@@ -169,7 +169,7 @@ impl PickerDelegate for SymbolContextPickerDelegate {
         _window: &mut Window,
         _: &mut Context<Picker<Self>>,
     ) -> Option<Self::ListItem> {
-        let mat = &self.matches[ix];
+        let mat = &self.matches.get(ix)?;
 
         Some(ListItem::new(ix).inset(true).toggle_state(selected).child(
             render_symbol_context_entry(ElementId::named_usize("symbol-ctx-picker", ix), mat),

crates/agent_ui/src/context_picker/thread_context_picker.rs

@@ -220,7 +220,7 @@ impl PickerDelegate for ThreadContextPickerDelegate {
         _window: &mut Window,
         cx: &mut Context<Picker<Self>>,
     ) -> Option<Self::ListItem> {
-        let thread = &self.matches[ix];
+        let thread = &self.matches.get(ix)?;
 
         Some(ListItem::new(ix).inset(true).toggle_state(selected).child(
             render_thread_context_entry(thread, self.context_store.clone(), cx),