fix(genai): cache GenerativeService(Async)Clients

[ccurme]

wip: struggling to properly clean up async clients when cached:

Error in sys.excepthook:

Original exception was:
WARNING: All log messages before absl::InitializeLog() is called are written to STDERR
E0000 00:00:1750261203.321942  822808 init.cc:232] grpc_wait_for_shutdown_with_timeout() timed out.