AgentENV fails on kernels affected by the ublk ADD_DEV initialization regression

## Summary

AgentENV cannot create ublk-backed block devices on kernels affected by a ublk initialization-order regression. The first `UBLK_U_CMD_ADD_DEV` operation can trigger a kernel NULL-pointer dereference, after which the command never completes and AgentENV cannot create or run sandboxes.

It affects any kernel containing:

- `529d4d632788` (`ublk: implement NUMA-aware memory allocation`)
- without its prerequisite `011af85ccd87` (`ublk: reorder tag_set initialization before queue allocation`)

## Known affected kernels

- Upstream stable Linux `6.18.4`
- Upstream stable Linux `6.18.5`
- Arch Linux `6.18.4-arch1-1`
- Ubuntu `6.17.0-1015-azure`
- Ubuntu `6.17.0-1018-azure` is also [reported as still affected](https://github.com/actions/runner-images/issues/14175#issuecomment-5054177083)

Other downstream kernels may be affected if they backported the NUMA change without the prerequisite reorder.

## Known fixed or unaffected kernels

- Upstream stable Linux `6.18.6` and later `6.18.y` releases contain the fix
- Kernels containing both commits are not affected
- Older kernels without the NUMA change are not affected by this specific regression

## References

- [Ubuntu Launchpad bug #2154635](https://bugs.launchpad.net/ubuntu/+source/linux-azure/+bug/2154635)
- [Upstream ublksrv issue #174](https://github.com/ublk-org/ublksrv/issues/174)
- [Linux 6.18.6 fix](https://cdn.kernel.org/pub/linux/kernel/v6.x/ChangeLog-6.18.6)

Start with the affected kernel versions and the referenced ublk commits, upstream issue, and 6.18.6 changelog. The issue names no AgentENV file or test and does not specify the desired project change, so the implementation path and definition of done need clarification.

Written by the indexing model from the issue text.