assistant_tools: Reduce allocations (#30776)

Another batch of allocation savings. Noteworthy ones are
`find_path_tool.rs` where one clone of *all* found matches was saved and
`web_tool_search.rs` where the tooltip no longer clones the entire url
on every hover.

I'd also like to propose using `std::borrow::Cow` a lot more around the
codebase instead of Strings. There are hundreds if not 1000+ clones that
can be saved pretty regularly simply by switching to Cow. ´Cow´'s are
likely not used because they aren't compatible with futures and because
it could cause lifetime bloat. However if we use `Cow<'static, str>`
(static lifetime) for when we need to pass them into futures, we could
save a TON of allocations for `&'static str`. Additionally I often see
structs being created using `String`'s just to be deserialized
afterwards, which only requires a reference.

Release Notes:

- N/A

crates/assistant_tools/src/fetch_tool.rs

@@ -1,6 +1,6 @@
-use std::cell::RefCell;
 use std::rc::Rc;
 use std::sync::Arc;
+use std::{borrow::Cow, cell::RefCell};
 
 use crate::schema::json_schema_for;
 use anyhow::{Context as _, Result, anyhow, bail};
@@ -39,10 +39,11 @@ impl FetchTool {
     }
 
     async fn build_message(http_client: Arc<HttpClientWithUrl>, url: &str) -> Result<String> {
-        let mut url = url.to_owned();
-        if !url.starts_with("https://") && !url.starts_with("http://") {
-            url = format!("https://{url}");
-        }
+        let url = if !url.starts_with("https://") && !url.starts_with("http://") {
+            Cow::Owned(format!("https://{url}"))
+        } else {
+            Cow::Borrowed(url)
+        };
 
         let mut response = http_client.get(&url, AsyncBody::default(), true).await?;
 
@@ -156,8 +157,7 @@ impl Tool for FetchTool {
 
         let text = cx.background_spawn({
             let http_client = self.http_client.clone();
-            let url = input.url.clone();
-            async move { Self::build_message(http_client, &url).await }
+            async move { Self::build_message(http_client, &input.url).await }
         });
 
         cx.foreground_executor()