assistant2: Add ability to fetch URLs as context (#21988)

This PR adds the ability to fetch URLs as context in Assistant2.

In the picker we use the search area as an input for the user to enter
the URL they wish to fetch:

<img width="1159" alt="Screenshot 2024-12-13 at 2 45 41 PM"
src="https://github.com/user-attachments/assets/b3b20648-2c22-4509-b592-d0291d25b202"
/>

<img width="1159" alt="Screenshot 2024-12-13 at 2 45 47 PM"
src="https://github.com/user-attachments/assets/7e6bab2d-2731-467f-9781-130c6e4ea5cf"
/>

Release Notes:

- N/A

crates/assistant2/src/thread.rs

@@ -193,12 +193,19 @@ impl Thread {
 
             if let Some(context) = self.context_for_message(message.id) {
                 let mut file_context = String::new();
+                let mut fetch_context = String::new();
 
                 for context in context.iter() {
                     match context.kind {
                         ContextKind::File => {
                             file_context.push_str(&context.text);
-                            file_context.push_str("\n");
+                            file_context.push('\n');
+                        }
+                        ContextKind::FetchedUrl => {
+                            fetch_context.push_str(&context.name);
+                            fetch_context.push('\n');
+                            fetch_context.push_str(&context.text);
+                            fetch_context.push('\n');
                         }
                     }
                 }
@@ -209,6 +216,11 @@ impl Thread {
                     context_text.push_str(&file_context);
                 }
 
+                if !fetch_context.is_empty() {
+                    context_text.push_str("The following fetched results are available\n");
+                    context_text.push_str(&fetch_context);
+                }
+
                 request_message
                     .content
                     .push(MessageContent::Text(context_text))