Add option for code context menu items to have dynamic width (#37404)

Follow up to https://github.com/zed-industries/zed/pull/30598

This PR introduces the `display_options` field in the
`CompletionResponse`, allowing a code context menu width to be
dynamically dictated based on its larger item. This will allow us to
have the @-mentions and slash commands completion menus in the agent
panel not be bigger than it needs to be. It may also be relevant/useful
in the future for other use cases.

For now, we set all instances of code context menus to use a fixed
width, as defined in the PR linked above, which means this PR shouldn't
cause any visual change.

Release Notes:

- N/A

Co-authored-by: Michael Sloan <mgsloan+github@gmail.com>

crates/project/src/project.rs

@@ -574,11 +574,23 @@ impl std::fmt::Debug for Completion {
 /// Response from a source of completions.
 pub struct CompletionResponse {
     pub completions: Vec<Completion>,
+    pub display_options: CompletionDisplayOptions,
     /// When false, indicates that the list is complete and so does not need to be re-queried if it
     /// can be filtered instead.
     pub is_incomplete: bool,
 }
 
+#[derive(Default)]
+pub struct CompletionDisplayOptions {
+    pub dynamic_width: bool,
+}
+
+impl CompletionDisplayOptions {
+    pub fn merge(&mut self, other: &CompletionDisplayOptions) {
+        self.dynamic_width = self.dynamic_width && other.dynamic_width;
+    }
+}
+
 /// Response from language server completion request.
 #[derive(Clone, Debug, Default)]
 pub(crate) struct CoreCompletionResponse {