cuda.core: PinnedMemoryResource.is_device_accessible returns True without granting device access to the pool #3205

PinnedMemoryResource.is_device_accessible returns True unconditionally, and its docstring says the resource "provides device-accessible buffers" (cuda_core/cuda/core/_memory/_pinned_memory_resource.pyx, the is_device_accessible property). The resource creates its pool with CUmemPoolProps only and never calls cuMemPoolSetAccess.

The driver documents the default differently. From cuMemPoolCreate: "In the case of pools created with CU_MEM_LOCATION_TYPE_HOST_NUMA or CU_MEM_LOCATION_TYPE_HOST, their default accessibility will be from the host CPU." Device access to such a pool is granted with cuMemPoolSetAccess.

Nothing in the test suite settles which is right in practice: every kernel launch on pinned memory uses LegacyPinnedMemoryResource, and the pool-backed tests only copy on the host.

Asks:

1. A test that launches a kernel which reads and writes a PinnedMemoryResource buffer, on the allocating device, with and without numa_id set.
2. If the kernel faults: grant access for the device at pool creation (cuMemPoolSetAccess with CU_MEM_ACCESS_FLAGS_PROT_READWRITE for the current device, or expose the choice), and keep is_device_accessible as True. If it does not fault: keep the behavior, and make the docstring say what the access rule is, so it does not promise more than the pool provides.

Found while reviewing #2713, whose docstring text for allocate touched the same question.