fix: Implement get_or_create_default_folder for user folder management (#6090)

* feat: implement get_or_create_default_folder function to ensure default folder exists for users

* refactor: replace create_default_folder_if_it_doesnt_exist with get_or_create_default_folder for user folder creation

* test: add unit tests for get_or_create_default_folder function

* ✨ (generalBugs-shard-10.spec.ts): refactor test script to improve readability and maintainability by chaining actions on page elements instead of using separate lines for each action

---------

Co-authored-by: cristhianzl <cristhian.lousa@gmail.com>

src/backend/tests/unit/initial_setup/test_setup_functions.py

@@ -0,0 +1,52 @@
+import asyncio
+from uuid import uuid4
+
+import pytest
+from langflow.initial_setup.setup import DEFAULT_FOLDER_NAME, get_or_create_default_folder, session_scope
+from langflow.services.database.models.folder.model import FolderRead
+
+
+@pytest.mark.usefixtures("client")
+async def test_get_or_create_default_folder_creation() -> None:
+    """Test that a default folder is created for a new user.
+
+    This test verifies that when no default folder exists for a given user,
+    get_or_create_default_folder creates one with the expected name and assigns it an ID.
+    """
+    test_user_id = uuid4()
+    async with session_scope() as session:
+        folder = await get_or_create_default_folder(session, test_user_id)
+        assert folder.name == DEFAULT_FOLDER_NAME, "The folder name should match the default."
+        assert hasattr(folder, "id"), "The folder should have an 'id' attribute after creation."
+
+
+@pytest.mark.usefixtures("client")
+async def test_get_or_create_default_folder_idempotency() -> None:
+    """Test that subsequent calls to get_or_create_default_folder return the same folder.
+
+    The function should be idempotent such that if a default folder already exists,
+    calling the function again does not create a new one.
+    """
+    test_user_id = uuid4()
+    async with session_scope() as session:
+        folder_first = await get_or_create_default_folder(session, test_user_id)
+        folder_second = await get_or_create_default_folder(session, test_user_id)
+        assert folder_first.id == folder_second.id, "Both calls should return the same folder instance."
+
+
+@pytest.mark.usefixtures("client")
+async def test_get_or_create_default_folder_concurrent_calls() -> None:
+    """Test concurrent invocations of get_or_create_default_folder.
+
+    This test ensures that when multiple concurrent calls are made for the same user,
+    only one default folder is created, demonstrating idempotency under concurrent access.
+    """
+    test_user_id = uuid4()
+
+    async def get_folder() -> FolderRead:
+        async with session_scope() as session:
+            return await get_or_create_default_folder(session, test_user_id)
+
+    results = await asyncio.gather(get_folder(), get_folder(), get_folder())
+    folder_ids = {folder.id for folder in results}
+    assert len(folder_ids) == 1, "Concurrent calls must return a single, consistent folder instance."