refactor: move `decrypt_message()` code `ConversationGuard` (2/2) [WPB-15583] #1962

Job	Run time
web benchmarks with bencher	2m 29s
rust benchmarks with bencher (commit)	12m 1s
rust benchmarks with bencher (encryption)	7m 12s
rust benchmarks with bencher (key_package)	5m 12s
rust benchmarks with bencher (create_group)	6m 35s
rust benchmarks with bencher (mls_proteus)	6m 30s
rust benchmarks with bencher (transaction)	5m 16s
	45m 15s

Provide feedback