tests: optimize test durations and refactor test fixtures (#3713)

* Refactor test fixtures and imports for optimization

- Reordered and optimized imports in `conftest.py`.
- Changed return statements to yield in `basic_graph` and `create_webhook` fixtures for better resource management.
- Added cleanup step to `create_webhook` fixture to delete created webhook after test completion.

* Refactor imports and add placeholder client fixture in test_ChatOllama_component.py

* Add pytest fixture 'client' to test_graph.py

* Add pytest fixture 'client' to test_callback_graph.py for test optimization

* Add 'unit_tests_looponfail' target to Makefile for running unit tests with loop-on-fail option

* Optimize test durations and update test cases in various modules

src/backend/tests/unit/graph/test_graph.py

@@ -25,6 +25,11 @@ from langflow.utils.payload import get_root_vertex
 # BASIC_EXAMPLE_PATH, COMPLEX_EXAMPLE_PATH, OPENAPI_EXAMPLE_PATH
 
 
+@pytest.fixture
+def client():
+    pass
+
+
 @pytest.fixture
 def sample_template():
     return {