community: correct return type of get_files_from_directory in github tool (#27885)

### About:
- **Description:** the _get_files_from_directory_ method return a
string, but it's used in other methods that expect a List[str]
- **Issue:** None
- **Dependencies:** None

This pull request import a new method _list_files_ with the old logic of
_get_files_from_directory_, but it return a List[str] at the end.
The behavior of _ get_files_from_directory_ is not changed.

libs/community/langchain_community/utilities/github.py

@@ -213,7 +213,7 @@ class GitHubAPIWrapper(BaseModel):
             )
             for content in contents:
                 if content.type == "dir":
-                    files.extend(self.get_files_from_directory(content.path))
+                    files.extend(self._list_files(content.path))
                 else:
                     files.append(content.path)
 
@@ -324,7 +324,7 @@ class GitHubAPIWrapper(BaseModel):
             )
             for content in contents:
                 if content.type == "dir":
-                    files.extend(self.get_files_from_directory(content.path))
+                    files.extend(self._list_files(content.path))
                 else:
                     files.append(content.path)
 
@@ -351,20 +351,24 @@ class GitHubAPIWrapper(BaseModel):
         """
         from github import GithubException
 
-        files: List[str] = []
         try:
-            contents = self.github_repo_instance.get_contents(
-                directory_path, ref=self.active_branch
-            )
+            return str(self._list_files(directory_path))
         except GithubException as e:
             return f"Error: status code {e.status}, {e.message}"
 
+    def _list_files(self, directory_path: str) -> List[str]:
+        files: List[str] = []
+
+        contents = self.github_repo_instance.get_contents(
+            directory_path, ref=self.active_branch
+        )
+
         for content in contents:
             if content.type == "dir":
-                files.extend(self.get_files_from_directory(content.path))
+                files.extend(self._list_files(content.path))
             else:
                 files.append(content.path)
-        return str(files)
+        return files
 
     def get_issue(self, issue_number: int) -> Dict[str, Any]:
         """