Update SandboxMixin

[DamianB-BitFlipper]

Description

Type of Change

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)
• Documentation update
• Test improvement

Testing

• All existing tests pass when running uv run pytest locally.
• New tests have been added to cover the changes

Checklist

• My code follows the style guidelines of this project as outlined in AGENTS.md
• I have performed a self-review of my own code
• I have commented my code, particularly in hard-to-understand areas
• I have made corresponding changes to the documentation
• My changes generate no new warnings
• Any dependent changes have been merged and published

Additional Notes

---

Note

Low Risk
Documentation-only change; no runtime behavior or APIs are modified.

Overview
Updates verifiers/envs/experimental/README.md to document SandboxMixin support for both container and VM sandboxes, including how to request VMs (and GPU VMs) via CreateSandboxRequest.

Clarifies current feature parity/limitations by noting that port exposure and SSH are container-only while common operations (file I/O, background jobs, cleanup) behave the same across sandbox types.

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[mikasenghaas]

ah but it looks like we alr support the vm attr bc its part of CreateSandboxRequest? in that case, i dont acc think we need any additional wiring here