[BUG] cudf-polars error in a MIG sandbox

(Claude-generated bug report follows)

# cudf-polars: `NVMLError_NoPermission` on MIG-partitioned GPUs

**Status:** present in `cudf-polars-cu12` 26.4.0 with Polars 1.40.1.

`pl.GPUEngine` raises `polars.exceptions.ComputeError: 'cuda' conversion
failed: NVMLError_NoPermission: Insufficient Permissions` on every
`.collect()` when the visible device is a MIG slice.

## Environment

- Driver: `580.126.20` (CUDA 13.0)
- GPU: NVIDIA RTX PRO 6000 Blackwell Server Edition with MIG mode
enabled (`1g.24gb` slices).
- `CUDA_VISIBLE_DEVICES=MIG-` pointed at one slice.
- `cudf-polars-cu12==26.4.0`, `polars==1.40.1`, Python 3.13.

Plain `cudf.DataFrame(...).sum()` works in the same environment, so
this is specific to the cudf-polars callback path, not RMM/cuDF
itself.

## Repro

```python
import polars as pl
gpu = pl.GPUEngine(raise_on_fail=True, executor="streaming")
df = pl.DataFrame({"a": [1, 2, 3, 4, 5]})
df.lazy().sum().collect(engine=gpu)
# polars.exceptions.ComputeError:
# 'cuda' conversion failed: NVMLError_NoPermission: Insufficient Permissions
```

Run with `CUDA_VISIBLE_DEVICES=MIG-`.

## Root cause

`cudf_polars/utils/config.py::get_device_handle` (lines 70–92 in 26.4.0)
detects when the visible device is a MIG slice and **upgrades** the
NVML handle from the MIG slice to its parent device:

```python
handle = pynvml.nvmlDeviceGetHandleByUUID(str.encode(index))
if pynvml.nvmlDeviceIsMigDeviceHandle(handle):
# Additionally get parent device handle
# if the device itself is a MIG instance
handle = pynvml.nvmlDeviceGetDeviceHandleFromMigDeviceHandle(handle)
```

`get_total_device_memory` then calls
`pynvml.nvmlDeviceGetMemoryInfo(parent_handle)` to size the RMM pool.
From inside a MIG sandbox, querying the parent device's memory is
privileged — it raises `NVMLError_NoPermission`, not
`NVMLError_NotSupported`, so the existing `except
pynvml.NVMLError_NotSupported` clause doesn't catch it. The exception
propagates, the cudf-polars callback fails to register, and Polars
surfaces the error as a `ComputeError` at `.collect()` time.

The upgrade was presumably added to support nested MIG handles or to
get a "canonical" handle, but on the GetMemoryInfo path it's the wrong
direction: the MIG handle itself is happy to answer GetMemoryInfo, and
the value returned (the slice's memory, not the full parent's) is the
correct number to use when sizing RMM pool — RMM is allocating inside
the slice, not the parent.

## Verification

Removing the upgrade (using the MIG handle directly) makes the bug go
away:

```diff
handle = pynvml.nvmlDeviceGetHandleByUUID(str.encode(index))
- if pynvml.nvmlDeviceIsMigDeviceHandle(handle):
- # Additionally get parent device handle
- # if the device itself is a MIG instance
- handle = pynvml.nvmlDeviceGetDeviceHandleFromMigDeviceHandle(handle)
```

After the patch, the repro above succeeds and returns
`shape: (1, 1) ┌─ a ─┐ │ 15 │ └─────┘`.

## Suggested fix

Drop the parent-handle upgrade in `get_device_handle`. If the upgrade
was needed somewhere else, scope it to that call rather than caching
the parent handle for everyone (`get_device_handle` is
`@functools.cache`d, so any caller that reuses it inherits the
parent-handle problem).

A minimal alternative is to broaden the exception clause in
`get_total_device_memory` so `NVMLError_NoPermission` also returns
`None` (treating "we can't size the pool" as an absent device-memory
signal). That hides the bug in the cache-sizing path but doesn't fix
it for any future caller of `get_device_handle()`.

## Other affected configurations

Likely affects any MIG-mode GPU under cudf-polars. Tested only on
RTX PRO 6000 Blackwell, but the issue is structural in NVML's
permission model — `nvmlDeviceGetMemoryInfo` on the parent device
from inside a MIG sandbox is documented as restricted.

## Notes

- `cudf` proper (without the `-polars` callback) does not hit this
path; it sizes RMM differently.
- The benchmarks-side helper at
`cudf_polars/experimental/benchmarks/utils.py` has the same NVML
dance but catches `NVMLError_NotSupported` only, so it would
presumably hit the same wall on MIG. Not exercised in this repro
but worth fixing in the same PR.