Speed up list_directory tool (#851)

# Motivation

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# Content

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# Testing

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# Please check the following before marking your PR as ready for review

- [ ] I have added tests for my changes
- [ ] I have updated the documentation or added new documentation as
needed

Author: EdwardJXLi

src/codegen/extensions/tools/list_directory.py

@@ -140,9 +140,8 @@ def get_directory_info(dir_obj: Directory, current_depth: int) -> DirectoryInfo:
         """Helper function to get directory info recursively."""
         # Get direct files (always include files unless at max depth)
         all_files = []
-        for file in dir_obj.files(recursive=True):
-            if file.directory == dir_obj:
-                all_files.append(file.filepath.split("/")[-1])
+        for file_name in dir_obj.file_names:
+            all_files.append(file_name)
 
         # Get direct subdirectories
         subdirs = []

src/codegen/sdk/core/directory.py

@@ -196,6 +196,11 @@ def item_names(self, recursive: bool = False) -> list[str]:
         """
         return self._files + self._subdirectories
 
+    @property
+    def file_names(self) -> list[str]:
+        """Get a list of all file names in the directory."""
+        return self._files
+
     @property
     def tree(self) -> list[Self | TFile]:
         """Get a recursive list of all files and subdirectories in the directory.