Fix unused_container propagation

[saki7]

Compound parsers should accept BOTH unused_type and unused_container_type.

Note that their types are accepted as-is but the values are unconditionally converted to unused_type. This is currently the correct solution because the parsers which need unused_container_type actually converts unused_type to unused_container_type in their parse function. If we instead keep the unused_container instance, some underlying parsers might fail to select the correct overload which relies on unused_type (instead of unused_container_type).

[cppwarningnotifier]

Environment				C++23	C++26
x4	Clang	21	Debug	✅success	✅success
			Release	✅success	✅success
	GCC	14	Debug	✅success	✅success
			Release	✅success	✅success
	MSVC	2022	Debug	✅success	✅success
			Release	✅success	✅success
		2026	Debug	✅success	✅success
			Release	✅success	✅success