fix(cuda.core): declare cdef attrs for system event wrappers and decode packed gpu_id

[rluo8]

Description

Two issues were found when adding coverage tests (internal bug 6595639):

1. Missing cdef attributes on SystemEvent, SystemEvents, EventData, and DeviceAttributes. These are Cython cdef class types (no __dict__), so undeclared self._event_data / self._attributes assignments raise AttributeError. RegisteredSystemEvents.wait() therefore crashes as soon as a real bind/unbind event arrives.

2. SystemEvent.device type mismatch. NVML's packed int gpu_id (domain[31:16] | bus[15:8] | device[7:0]), which requires an NVML-style PCI bus ID string (NVML_DEVICE_PCI_BUS_ID_FMT). Decode at the call site via _pci_bus_id_from_gpu_id.

Adds regression tests for construction, packing decode, and end-to-end SystemEvent.device resolution against live pci_info.

Verification:

• B10: new regression tests 8 passed; live unbind of 0000:c1:00.0
  yields SystemEvents with gpu_id=0xC100 → 00000000:C1:00.0, and
  post-rebind SystemEvent.device resolves correctly
• A100 MIG: DeviceAttributes cdef fix verified
• System Event register/wait remains Linux-only (Windows/WSL/WOA skip)

[rluo8]

Hi @mdboom , this issue is about system event. Could you please help review it?
Thanks!

[github-actions]

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[mdboom]

What is the root issue this is trying to solve?

This is making these objects mutable and they definitely shouldn't be -- their intention is to return values from the NVML API, not to set things on them.

The gpu_id / pci_id bridge in _system_events.pyx added here makes sense, though.