Fix LangCache clear() 400 error by using flush() API #818

Job	Run time
Service Tests	4m 5s
docs	27s
Python 3.9 - redis-py 5.x [redis 6.2.6-v9]	2m 14s
Python 3.9 - redis-py 5.x [redis 8.0.2]	2m 14s
Python 3.9 - redis-py 5.x [redis latest]	2m 43s
Python 3.10 - redis-py 5.x [redis latest]	2m 52s
Python 3.10 - redis-py 5.x [redis 6.2.6-v9]	2m 10s
Python 3.9 - redis-py 6.x [redis 8.0.2]	2m 9s
Python 3.10 - redis-py 5.x [redis 8.0.2]	2m 26s
Python 3.10 - redis-py 6.x [redis latest]	4m 17s
Python 3.12 - redis-py 6.x [redis 6.2.6-v9]	5m 23s
Python 3.12 - redis-py 5.x [redis 6.2.6-v9]	2m 4s
Python 3.12 - redis-py 5.x [redis 8.0.2]	2m 12s
Python 3.10 - redis-py 6.x [redis 6.2.6-v9]	2m 26s
Python 3.12 - redis-py 6.x [redis latest]	4m 25s
Python 3.10 - redis-py 6.x [redis 8.0.2]	2m 13s
Python 3.9 - redis-py 6.x [redis 6.2.6-v9]	2m 14s
Python 3.12 - redis-py 6.x [redis 8.0.2]	2m 41s
Python 3.13 - redis-py 6.x [redis latest]	4m 42s
Python 3.13 - redis-py 5.x [redis 8.0.2]	2m 13s
Python 3.13 - redis-py 6.x [redis 6.2.6-v9]	2m 26s
Python 3.13 - redis-py 5.x [redis 6.2.6-v9]	2m 22s
Python 3.12 - redis-py 5.x [redis latest]	2m 15s
Python 3.13 - redis-py 6.x [redis 8.0.2]	2m 37s
Python 3.13 - redis-py 5.x [redis latest]	2m 14s
Python 3.9 - redis-py 6.x [redis latest]	4m 43s
	1h 12m 47s

Provide feedback