agent: Add websearch tool (#28621)

Staff only for now. We'll work on making this usable for non zed.dev
users later

Release Notes:

- N/A

---------

Co-authored-by: Antonio Scandurra <me@as-cii.com>
Co-authored-by: Danilo Leal <daniloleal09@gmail.com>
Co-authored-by: Marshall Bowers <git@maxdeviant.com>

crates/zed/src/zed.rs

@@ -4258,6 +4258,8 @@ mod tests {
                 app_state.fs.clone(),
                 cx,
             );
+            web_search::init(cx);
+            web_search_providers::init(app_state.client.clone(), cx);
             let prompt_builder = PromptBuilder::load(app_state.fs.clone(), false, cx);
             assistant::init(
                 app_state.fs.clone(),