Fix `IncompatibleReturnValueException` in `MessageFactoryTest` #2766

Job	Run time
php (8.1, false)	2m 3s
php (8.2, false)	2m 6s
php (8.3, false)	2m 8s
php (8.4, true, --ignore-platform-reqs)	11m 19s
packages / validate (8.1, ubuntu-latest, ast, json, grpc, false)	1m 11s
packages / validate (8.2, ubuntu-latest, ast, json, grpc, false)	53s
packages / validate (8.3, ubuntu-latest, ast, json, grpc, false)	53s
	20m 33s

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