Address PR 44074 sleep backend review comments

[simon-mo]

Purpose

Address review comments on vllm-project#44074 without opening another upstream PR against main:

• Move the new sleep-mode backend tests out of the top-level tests/ directory into tests/v1/worker/test_gpu_worker.py.
• Remove speculative/NCCL-specific capability methods from the generic SleepModeBackend abstraction, keeping only suspend/resume/state and platform support validation.

Duplicate-work check: vllm-project#44074 is the existing upstream PR for the sleep-mode backend abstraction; this helper PR is intentionally based on a local copy of that PR's pre-fix head so the diff contains only comment fixes. Also checked open upstream PRs for 44074 in:body and sleep mode backend abstraction; vllm-project#45398 depends on vllm-project#44074 rather than duplicating this helper change.

AI assistance was used to prepare these changes.

Test Plan

• .venv/bin/python -m pytest tests/v1/worker/test_gpu_worker.py -q
• .venv/bin/pre-commit run ruff-check --files vllm/device_allocator/sleep_mode_backend.py tests/v1/worker/test_gpu_worker.py

Test Result

• 11 passed, 16 warnings in 8.58s
• ruff check ... Passed

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