refactor: move `decrypt_message()` code `ConversationGuard` (2/2) [WPB-15583] #1961

Job	Run time
rust benchmarks with bencher (commit)	11m 48s
web benchmarks with bencher	2m 36s
rust benchmarks with bencher (encryption)	7m 15s
rust benchmarks with bencher (key_package)	5m 3s
rust benchmarks with bencher (create_group)	6m 29s
rust benchmarks with bencher (mls_proteus)	6m 12s
rust benchmarks with bencher (transaction)	5m 18s
	44m 41s

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