UCT/CUDA_IPC: Add POSIX FD handle type for same-machine VMM IPC (#11183)

* UCT/CUDA_IPC: Add POSIX FD handle type for same-machine VMM IPC

Add support for CUDA VMM allocations that expose POSIX file descriptor
handles (CU_MEM_HANDLE_TYPE_POSIX_FILE_DESCRIPTOR) as an IPC transport
in cuda_ipc. When a VMM allocation does not support fabric handles but
supports POSIX FD handles, the FD is exported along with a system_id
to identify the source machine. On the receiver side, system_id is
verified to ensure both peers are on the same machine, and the FD is
duplicated via pidfd_open/pidfd_getfd (Linux 5.6+) before importing
through cuMemImportFromShareableHandle.

This enables IPC for VMM-backed GPU memory on systems without fabric
support.

* UCT/CUDA_IPC: Verify same pid namespace for POSIX_FD