Add GET /internal/files. (#4295)

* Create internal route table.

* List files.

* Add GET /internal/files.

Retrieves list of files in models, output, and user directories.

* Refactor file names.

* Use typing_extensions for Python 3.8

* Fix tests.

* Remove print statements.

* Update README.

* Add output and user to valid directory test.

* Add missing type hints.

tests-unit/server/utils/file_operations_test.py

@@ -0,0 +1,42 @@
+import pytest
+from typing import List
+from api_server.utils.file_operations import FileSystemOperations, FileSystemItem, is_file_info
+
+@pytest.fixture
+def temp_directory(tmp_path):
+    # Create a temporary directory structure
+    dir1 = tmp_path / "dir1"
+    dir2 = tmp_path / "dir2"
+    dir1.mkdir()
+    dir2.mkdir()
+    (dir1 / "file1.txt").write_text("content1")
+    (dir2 / "file2.txt").write_text("content2")
+    (tmp_path / "file3.txt").write_text("content3")
+    return tmp_path
+
+def test_walk_directory(temp_directory):
+    result: List[FileSystemItem] = FileSystemOperations.walk_directory(str(temp_directory))
+    
+    assert len(result) == 5  # 2 directories and 3 files
+    
+    files = [item for item in result if item['type'] == 'file']
+    dirs = [item for item in result if item['type'] == 'directory']
+    
+    assert len(files) == 3
+    assert len(dirs) == 2
+    
+    file_names = {file['name'] for file in files}
+    assert file_names == {'file1.txt', 'file2.txt', 'file3.txt'}
+    
+    dir_names = {dir['name'] for dir in dirs}
+    assert dir_names == {'dir1', 'dir2'}
+
+def test_walk_directory_empty(tmp_path):
+    result = FileSystemOperations.walk_directory(str(tmp_path))
+    assert len(result) == 0
+
+def test_walk_directory_file_size(temp_directory):
+    result: List[FileSystemItem] = FileSystemOperations.walk_directory(str(temp_directory))
+    files = [item for item in result if is_file_info(item)]
+    for file in files:
+        assert file['size'] > 0  # Assuming all files have some content