If file is too big, provide the outline and suggest a follow-up tool (#28158)

<img width="622" alt="Screenshot 2025-04-05 at 5 48 14 PM"
src="https://github.com/user-attachments/assets/24b9c7d4-d3e2-4929-bca8-79db5b4e5748"
/>

Release Notes:

- The `read_files` tool now reads only the symbol outline files above a
certain size, to conserve context window space. Then it suggests that
the agent call `read_files` again with the relevant line ranges it saw
in the outline.

crates/assistant_tools/src/code_symbols_tool.rs

@@ -156,7 +156,7 @@ impl Tool for CodeSymbolsTool {
     }
 }
 
-async fn file_outline(
+pub async fn file_outline(
     project: Entity<Project>,
     path: String,
     action_log: Entity<ActionLog>,