Dedupe PromptBuilder construction (#23496)

This PR dedupes the construction of the `PromptBuilder`.

Previously this was constructed by both `assistant` and `assistant2`,
but now we construct it outside and pass it in.

Release Notes:

- N/A

crates/zed/src/zed.rs

@@ -3833,8 +3833,13 @@ mod tests {
                 app_state.fs.clone(),
                 cx,
             );
-            let prompt_builder =
-                assistant::init(app_state.fs.clone(), app_state.client.clone(), false, cx);
+            let prompt_builder = PromptBuilder::load(app_state.fs.clone(), false, cx);
+            assistant::init(
+                app_state.fs.clone(),
+                app_state.client.clone(),
+                prompt_builder.clone(),
+                cx,
+            );
             repl::init(app_state.fs.clone(), cx);
             repl::notebook::init(cx);
             tasks_ui::init(cx);