Code block evals (#29619)

Add a targeted eval for code block formatting, and revise the system
prompt accordingly.

### Eval before, n=8

<img width="728" alt="eval before"
src="https://github.com/user-attachments/assets/552b6146-3d26-4eaa-86f9-9fc36c0cadf2"
/>

### Eval after prompt change, n=8 (excluding the new evals, so just
testing the prompt change)

<img width="717" alt="eval after"
src="https://github.com/user-attachments/assets/c78c7a54-4c65-470c-b135-8691584cd73e"
/>

Release Notes:

- N/A

crates/assistant_tools/src/edit_file_tool.rs

@@ -174,6 +174,7 @@ impl Tool for EditFileTool {
                     "The `old_string` and `new_string` are identical, so no changes would be made."
                 ));
             }
+            let old_string = input.old_string.clone();
 
             let result = cx
                 .background_spawn(async move {
@@ -213,6 +214,21 @@ impl Tool for EditFileTool {
                             input.path.display()
                         )
                     } else {
+                        let old_string_with_buffer = format!(
+                            "old_string:\n\n{}\n\n-------file-------\n\n{}",
+                            &old_string,
+                            buffer.text()
+                        );
+                        let path = {
+                            use std::collections::hash_map::DefaultHasher;
+                            use std::hash::{Hash, Hasher};
+
+                            let mut hasher = DefaultHasher::new();
+                            old_string_with_buffer.hash(&mut hasher);
+
+                            PathBuf::from(format!("failed_tool_{}.txt", hasher.finish()))
+                        };
+                        std::fs::write(path, old_string_with_buffer).unwrap();
                         anyhow!("Failed to match the provided `old_string`")
                     }
                 })?;