Fix LangCache clear() 400 error by using flush() API #823

Job	Run time
docs	30s
Service Tests	3m 31s
Python 3.9 - redis-py 5.x [redis 8.0.2]	2m 18s
Python 3.9 - redis-py 6.x [redis 6.2.6-v9]	2m 8s
Python 3.9 - redis-py 6.x [redis latest]	4m 43s
Python 3.9 - redis-py 5.x [redis latest]	2m 47s
Python 3.10 - redis-py 5.x [redis 6.2.6-v9]	2m 17s
Python 3.10 - redis-py 6.x [redis latest]	4m 27s
Python 3.10 - redis-py 6.x [redis 6.2.6-v9]	2m 7s
Python 3.9 - redis-py 5.x [redis 6.2.6-v9]	2m 22s
Python 3.10 - redis-py 5.x [redis latest]	3m 11s
Python 3.10 - redis-py 6.x [redis 8.0.2]	2m 11s
Python 3.9 - redis-py 6.x [redis 8.0.2]	2m 34s
Python 3.12 - redis-py 5.x [redis 6.2.6-v9]	2m 18s
Python 3.12 - redis-py 5.x [redis latest]	2m 22s
Python 3.12 - redis-py 5.x [redis 8.0.2]	2m 46s
Python 3.12 - redis-py 6.x [redis 6.2.6-v9]	2m 12s
Python 3.10 - redis-py 5.x [redis 8.0.2]	2m 39s
Python 3.13 - redis-py 6.x [redis 6.2.6-v9]	2m 23s
Python 3.12 - redis-py 6.x [redis 8.0.2]	2m 44s
Python 3.12 - redis-py 6.x [redis latest]	4m 43s
Python 3.13 - redis-py 5.x [redis 8.0.2]	2m 24s
Python 3.13 - redis-py 5.x [redis 6.2.6-v9]	2m 30s
Python 3.13 - redis-py 5.x [redis latest]	2m 15s
Python 3.13 - redis-py 6.x [redis 8.0.2]	2m 49s
Python 3.13 - redis-py 6.x [redis latest]	5m 21s
	1h 12m 32s

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