Fix LangCache clear() 400 error by using flush() API #822

Job	Run time
Service Tests	4m 1s
docs	29s
Python 3.9 - redis-py 5.x [redis 8.0.2]	2m 11s
Python 3.9 - redis-py 5.x [redis 6.2.6-v9]	2m 10s
Python 3.9 - redis-py 5.x [redis latest]	2m 44s
Python 3.9 - redis-py 6.x [redis 8.0.2]	2m 44s
Python 3.9 - redis-py 6.x [redis 6.2.6-v9]	2m 14s
Python 3.9 - redis-py 6.x [redis latest]	4m 33s
Python 3.10 - redis-py 5.x [redis 8.0.2]	2m 22s
Python 3.10 - redis-py 5.x [redis 6.2.6-v9]	2m 18s
Python 3.10 - redis-py 5.x [redis latest]	3m 21s
Python 3.10 - redis-py 6.x [redis 6.2.6-v9]	2m 27s
Python 3.12 - redis-py 6.x [redis latest]	4m 15s
Python 3.12 - redis-py 6.x [redis 8.0.2]	2m 37s
Python 3.12 - redis-py 6.x [redis 6.2.6-v9]	2m 8s
Python 3.13 - redis-py 6.x [redis 8.0.2]	2m 42s
Python 3.10 - redis-py 6.x [redis 8.0.2]	3m 37s
Python 3.13 - redis-py 5.x [redis 8.0.2]	2m 28s
Python 3.13 - redis-py 6.x [redis 6.2.6-v9]	2m 2s
Python 3.13 - redis-py 5.x [redis 6.2.6-v9]	2m 11s
Python 3.10 - redis-py 6.x [redis latest]	4m 30s
Python 3.12 - redis-py 5.x [redis 6.2.6-v9]	2m 11s
Python 3.12 - redis-py 5.x [redis latest]	2m 21s
Python 3.13 - redis-py 6.x [redis latest]	8m 15s
Python 3.12 - redis-py 5.x [redis 8.0.2]	2m 11s
Python 3.13 - redis-py 5.x [redis latest]	2m 31s
	1h 15m 33s

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