assistant2: Wire up the directory context picker (#22582)

This PR wires up the functionality of the directory context picker.

Release Notes:

- N/A

---------

Co-authored-by: Agus <agus@zed.dev>

crates/assistant2/src/context_picker/file_context_picker.rs

@@ -316,7 +316,10 @@ impl PickerDelegate for FileContextPickerDelegate {
     }
 }
 
-fn codeblock_fence_for_path(path: Option<&Path>, row_range: Option<RangeInclusive<u32>>) -> String {
+pub(crate) fn codeblock_fence_for_path(
+    path: Option<&Path>,
+    row_range: Option<RangeInclusive<u32>>,
+) -> String {
     let mut text = String::new();
     write!(text, "```").unwrap();