[BUG]: Sanitizer errors in `MergeSort`

[gonidelis]

https://github.com/NVIDIA/cccl/actions/runs/23106493440/job/67117458183#step:4:2177

Weeklies reported uninitialized reads in DeviceMergeSort. Find source and either fix or suppress them

[gonidelis]

First indications show that this has to do with padded bytes not being initialized in custom_type_t objects

cccl/c2h/include/c2h/custom_type.h

Lines 13 to 22 in 832c8ba

	struct custom_type_state_t
	{
	std::size_t key{};
	std::size_t val{};
	};
	template <template <typename> class... Policies>
	class custom_type_t
	: public custom_type_state_t
	, public Policies<custom_type_t<Policies...>>...