agent: Fix bug in creating empty files (#31626)

Release Notes:

- NA

crates/assistant_tools/src/edit_agent/create_file_parser.rs

@@ -4,7 +4,7 @@ use std::cell::LazyCell;
 use util::debug_panic;
 
 const START_MARKER: LazyCell<Regex> = LazyCell::new(|| Regex::new(r"\n?```\S*\n").unwrap());
-const END_MARKER: LazyCell<Regex> = LazyCell::new(|| Regex::new(r"\n```\s*$").unwrap());
+const END_MARKER: LazyCell<Regex> = LazyCell::new(|| Regex::new(r"(^|\n)```\s*$").unwrap());
 
 #[derive(Debug)]
 pub enum CreateFileParserEvent {
@@ -184,6 +184,22 @@ mod tests {
         );
     }
 
+    #[gpui::test(iterations = 10)]
+    fn test_empty_file(mut rng: StdRng) {
+        let mut parser = CreateFileParser::new();
+        assert_eq!(
+            parse_random_chunks(
+                indoc! {"
+                    ```
+                    ```
+                "},
+                &mut parser,
+                &mut rng
+            ),
+            "".to_string()
+        );
+    }
+
     fn parse_random_chunks(input: &str, parser: &mut CreateFileParser, rng: &mut StdRng) -> String {
         let chunk_count = rng.gen_range(1..=cmp::min(input.len(), 50));
         let mut chunk_indices = (0..input.len()).choose_multiple(rng, chunk_count);