rust

refactor: move `decrypt_message()` code `ConversationGuard` (2/2) [WPB-15583] #5850

Job	Run time
check (fmt, --all -- --check)	15s
check (clippy --locked, -- -D warnings)	3m 14s
check (clippy --locked, -- -D warnings, --target wasm32-unknown-unknown)	1m 9s
check (check --locked, --tests)	3m 19s
check (check --locked, --tests, --target wasm32-unknown-unknown)	1m 19s
hack	1m 23s
proteus-test	1m 12s
test	5m 27s
wasm-test (keystore)	1m 5s
wasm-test (mls-provider)	1m 10s
wasm-test (crypto, -- e2e_identity)	1m 18s
wasm-test (crypto, -- mls)	2m 6s
wasm-test (crypto, -- group_store)	1m 25s
wasm-test (crypto, -- proteus)	1m 20s
wasm-test (keystore, --features proteus-keystore, -- proteus)	1m 8s
wasm-test (crypto, --features proteus,cryptobox-migrate, -- proteus)	1m 18s
	28m 8s

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