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

Makefile

@@ -148,6 +148,9 @@ else
 		$(args)
 endif
 
+unit_tests_looponfail:
+	@make unit_tests args="-f"
+
 integration_tests:
 	poetry run pytest src/backend/tests/integration \
 		--instafail -ra \