Chore: fix trait constrain for `external_state_machine_request<SM>()` #4346

Job	Run time
openraft-test-bench (nightly)	1m 0s
test-crate-openraft (stable)	51s
test-crate-openraft (nightly)	53s
cluster-benchmark (cluster_benchmark)	51s
examples (stable, memstore)	53s
lint	4m 0s
examples (stable, rocksstore)	5m 45s
openraft-feature-test (nightly)	54s
examples (stable, raft-kv-memstore)	2m 36s
openraft-feature-test (nightly, serde)	52s
examples (stable, raft-kv-memstore-grpc)	1m 54s
rt-monoio	43s
openraft-feature-test (nightly, serde,singlethreaded)	49s
examples (stable, raft-kv-memstore-network-v2)	56s
examples (stable, raft-kv-memstore-opendal-snapshot-data)	1m 18s
stores (stores/memstore)	54s
test-crate-tests (stable, 0)	3m 14s
examples (stable, raft-kv-memstore-singlethreaded)	1m 2s
test-crate-tests (nightly, 30)	3m 13s
examples (stable, raft-kv-rocksdb)	7m 29s
Build (nightly, bench,serde,bt,singlethreaded)	54s
test-crate-tests (nightly, 0, single-term-leader)	2m 56s
examples (nightly, memstore)	1m 3s
examples (nightly, rocksstore)	10m 19s
examples (nightly, raft-kv-memstore)	2m 25s
tests-feature-test (nightly)	3m 0s
examples (nightly, raft-kv-memstore-grpc)	1m 47s
examples (nightly, raft-kv-memstore-network-v2)	1m 6s
tests-feature-test (nightly, single-term-leader)	2m 57s
examples (nightly, raft-kv-memstore-opendal-snapshot-data)	1m 28s
examples (nightly, raft-kv-memstore-singlethreaded)	1m 6s
examples (nightly, raft-kv-rocksdb)	11m 32s
	1h 20m 40s

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