assistant2: Background load of context + prep for refresh + efficiency (#22935)

* Now loads context on background threads.

- For file and directory context, buffer ropes can be shared between
threads as they are immutable. This allows for traversal and
accumulation of buffer text on a background thread.

- For url context, the request, parsing, and rendering is now done on a
background thread.

* Prepares for support of buffer reload by individually storing the text
of directory buffers.

* Avoids some string copying / redundant strings.

- When attaching message context, no longer builds a string for each
context type.

- For directory context, does not build a `SharedString` for the full
text, instead has a slice of `SharedString` chunks which are then
directly appended to the message context.

- Building a fenced codeblock for a buffer now computes a precise
capacity in advance.

Release Notes:

- N/A

crates/assistant2/src/context_picker/fetch_context_picker.rs

@@ -81,13 +81,12 @@ impl FetchContextPickerDelegate {
         }
     }
 
-    async fn build_message(http_client: Arc<HttpClientWithUrl>, url: &str) -> Result<String> {
-        let prefixed_url = if !url.starts_with("https://") && !url.starts_with("http://") {
-            Some(format!("https://{url}"))
+    async fn build_message(http_client: Arc<HttpClientWithUrl>, url: String) -> Result<String> {
+        let url = if !url.starts_with("https://") && !url.starts_with("http://") {
+            format!("https://{url}")
         } else {
-            None
+            url
         };
-        let url = prefixed_url.as_deref().unwrap_or(url);
 
         let mut response = http_client.get(&url, AsyncBody::default(), true).await?;
 
@@ -196,7 +195,10 @@ impl PickerDelegate for FetchContextPickerDelegate {
         let url = self.url.clone();
         let confirm_behavior = self.confirm_behavior;
         cx.spawn(|this, mut cx| async move {
-            let text = Self::build_message(http_client, &url).await?;
+            let text = cx
+                .background_executor()
+                .spawn(Self::build_message(http_client, url.clone()))
+                .await?;
 
             this.update(&mut cx, |this, cx| {
                 this.delegate

crates/assistant2/src/context_picker/file_context_picker.rs

@@ -201,7 +201,7 @@ impl PickerDelegate for FileContextPickerDelegate {
         let Some(task) = self
             .context_store
             .update(cx, |context_store, cx| {
-                context_store.add_file(project_path, cx)
+                context_store.add_file_from_path(project_path, cx)
             })
             .ok()
         else {