assistant_tools: Add fetch tool (#26999)

This PR adds a new `fetch` tool to the set of tools the Assistant has
available.

This tool accepts a URL and fetches the content as Markdown.

<img width="1394" alt="Screenshot 2025-03-18 at 11 52 21 AM"
src="https://github.com/user-attachments/assets/e5bcde14-a0dd-4835-9d42-8f45def68f4d"
/>

<img width="1394" alt="Screenshot 2025-03-18 at 11 52 37 AM"
src="https://github.com/user-attachments/assets/3bcce4f5-f61b-40d7-8b30-2c673ce3c06a"
/>

Release Notes:

- N/A

crates/assistant_eval/src/headless_assistant.rs

@@ -163,7 +163,7 @@ pub fn init(cx: &mut App) -> Arc<HeadlessAppState> {
     language::init(cx);
     language_model::init(client.clone(), cx);
     language_models::init(user_store.clone(), client.clone(), fs.clone(), cx);
-    assistant_tools::init(cx);
+    assistant_tools::init(client.http_client().clone(), cx);
     context_server::init(cx);
     let stdout_is_a_pty = false;
     let prompt_builder = PromptBuilder::load(fs.clone(), stdout_is_a_pty, cx);