Add an eval binary that evaluates our semantic index against CodeSearchNet (#17375)

This PR is the beginning of an evaluation framework for our AI features.
Right now, we're evaluating our semantic search feature against the
[CodeSearchNet](https://github.com/github/CodeSearchNet) code search
dataset. This dataset is very limited (for the most part, only 1 known
good search result per repo) but it has surfaced some problems with our
search already.

Release Notes:

- N/A

---------

Co-authored-by: Jason <jason@zed.dev>
Co-authored-by: Jason Mancuso <7891333+jvmncs@users.noreply.github.com>
Co-authored-by: Nathan <nathan@zed.dev>
Co-authored-by: Richard <richard@zed.dev>

crates/assistant/src/assistant_panel.rs

@@ -3282,7 +3282,7 @@ impl ContextEditor {
 
                     let fence = codeblock_fence_for_path(
                         filename.as_deref(),
-                        Some(selection.start.row..selection.end.row),
+                        Some(selection.start.row..=selection.end.row),
                     );
 
                     if let Some((line_comment_prefix, outline_text)) =