Move system memory support check to runtime_capabilities.hpp

[bdice]

Problem

system_memory_resource needs a runtime support check for system allocated memory support using cudaDevAttrPageableMemoryAccess. That check currently lives with the memory resource implementation, while similar CUDA/runtime feature probes live in rmm/detail/runtime_capabilities.hpp.

Tests also need to skip system memory resource coverage when the runtime capability is unavailable. Keeping this probe as a memory-resource detail helper leaks resource implementation details into tests.

Proposed Solution

Add a system memory capability probe to rmm/detail/runtime_capabilities.hpp, alongside existing probes such as runtime_async_alloc, concurrent_managed_access, and device_integrated_memory.

Then update system_memory_resource and tests to use that runtime capability API instead of a memory-resource detail helper.

Acceptance Criteria

• System memory support is checked through rmm/detail/runtime_capabilities.hpp.
• system_memory_resource uses the shared runtime capability probe.
• System memory tests use the shared runtime capability probe for skips.
• No rmm::mr::detail::is_system_memory_supported helper remains necessary.
• Existing system memory resource tests pass.