Don't send contents of large @mention-ed files (#38032)

<img width="598" height="311" alt="Screenshot 2025-09-11 at 9 39 12 PM"
src="https://github.com/user-attachments/assets/b526e648-37cf-4412-83a0-42037b9fc94d"
/>

This is for both ACP and the regular agent. Previously we would always
include the whole file, which can easily blow the context window on huge
files.

Release Notes:

- When `@mention`ing large files, the Agent Panel now send an outline of
the file instead of the whole thing.

crates/agent/src/context.rs

@@ -6,7 +6,7 @@ use futures::future;
 use futures::{FutureExt, future::Shared};
 use gpui::{App, AppContext as _, ElementId, Entity, SharedString, Task};
 use icons::IconName;
-use language::{Buffer, ParseStatus};
+use language::Buffer;
 use language_model::{LanguageModelImage, LanguageModelRequestMessage, MessageContent};
 use project::{Project, ProjectEntryId, ProjectPath, Worktree};
 use prompt_store::{PromptStore, UserPromptId};
@@ -191,45 +191,19 @@ impl FileContextHandle {
         let buffer = self.buffer.clone();
 
         cx.spawn(async move |cx| {
-            // For large files, use outline instead of full content
-            if rope.len() > outline::AUTO_OUTLINE_SIZE {
-                // Wait until the buffer has been fully parsed, so we can read its outline
-                if let Ok(mut parse_status) =
-                    buffer.read_with(cx, |buffer, _| buffer.parse_status())
-                {
-                    while *parse_status.borrow() != ParseStatus::Idle {
-                        parse_status.changed().await.log_err();
-                    }
+            let buffer_content =
+                outline::get_buffer_content_or_outline(buffer.clone(), Some(&full_path), &cx)
+                    .await
+                    .unwrap_or_else(|_| outline::BufferContent {
+                        text: rope.to_string(),
+                        is_outline: false,
+                    });
 
-                    if let Ok(snapshot) = buffer.read_with(cx, |buffer, _| buffer.snapshot()) {
-                        let items = snapshot
-                            .outline(None)
-                            .items
-                            .into_iter()
-                            .map(|item| item.to_point(&snapshot));
-
-                        if let Ok(outline_text) =
-                            outline::render_outline(items, None, 0, usize::MAX).await
-                        {
-                            let context = AgentContext::File(FileContext {
-                                handle: self,
-                                full_path,
-                                text: outline_text.into(),
-                                is_outline: true,
-                            });
-                            return Some((context, vec![buffer]));
-                        }
-                    }
-                }
-            }
-
-            // Fallback to full content if we couldn't build an outline
-            // (or didn't need to because the file was small enough)
             let context = AgentContext::File(FileContext {
                 handle: self,
                 full_path,
-                text: rope.to_string().into(),
-                is_outline: false,
+                text: buffer_content.text.into(),
+                is_outline: buffer_content.is_outline,
             });
             Some((context, vec![buffer]))
         })