tests: update tests to use httpx.AsyncClient (#3984)

* Add async support and dependencies to pyproject.toml files

- Added `asgi-lifespan>=2.1.0` to dependencies.
- Configured `asyncio_mode` and `asyncio_default_fixture_loop_scope` for pytest.
- Updated `tool.uv` section with `asgi-lifespan` in dev-dependencies.

* Convert test fixtures to async and use AsyncClient for HTTP requests

* Handle 'ImportFrom' nodes in AST validation to support module attribute imports

* Convert test cases to use async HTTP client

- Updated test cases in `test_database.py`, `test_endpoints.py`, `test_user.py`, `test_variable.py`, `test_files.py`, `test_chat_endpoint.py`, `test_misc.py`, `test_messages_endpoints.py`, `test_api_key.py`, `test_webhook.py`, and `test_login.py` to use `httpx.AsyncClient` instead of `fastapi.TestClient`.
- Modified test functions to be asynchronous and use `await` for HTTP requests.
- Adjusted fixtures and helper functions to support asynchronous operations.
- Ensured consistency in endpoint paths and request methods across all test cases.

* Refactor string concatenation to f-string in test_chat_endpoint.py

* [autofix.ci] apply automated fixes

* Refactor import validation to use pattern matching for AST nodes

* Set `startup_timeout` and `shutdown_timeout` to `None` in `LifespanManager` for test files.

* Convert test functions to async in `test_messages_endpoints.py`

* Add `api_key_required` marker to assistant component tests

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Co-authored-by: autofix-ci[bot] <114827586+autofix-ci[bot]@users.noreply.github.com>

src/backend/tests/unit/test_cli.py

@@ -2,6 +2,7 @@ from pathlib import Path
 from tempfile import tempdir
 
 import pytest
+
 from langflow.__main__ import app
 from langflow.services import deps