[Core] Add container snapshot configuration and Worker lifecycle hooks (1/3)

[lbr711]

Purpose

This is stage 1 of the container snapshot support proposed in #54940. It adds
the backend-neutral configuration and extension points required by the engine
restore lifecycle introduced in the later stages.

The PR includes:

• opt-in SnapshotConfig and --snapshot-config parsing;
• an optional snapshot_metadata path for a mounted JSON metadata file used
  during restore;
• a platform hook for restore detection and a forced local-address probe that
  bypasses stale cached addresses;
• reservation of two additional data-parallel ports for reconstructing the
  EngineCore and device world communication groups after restore;
• executor-to-worker suspend, device_unlock, and resume lifecycle
  interfaces for device plugins;
• communication cleanup extension points used by device backends;
• snapshot-only persistence of the rotary cache.

Backend-specific device memory, communication-group, model-runner, and model
state restoration remains inside device plugins. vLLM does not expose
model-module snapshot hooks.

This stage does not implement EngineCore restore orchestration or expose HTTP
lifecycle endpoints.

Test Plan

VLLM_TARGET_DEVICE=cpu python -m pytest -q \
  tests/snapshot/test_config.py \
  tests/snapshot/test_utils.py \
  tests/entrypoints/launchers/test_cli_args.py \
  -k 'snapshot or test_is_restore'

python -m pytest -q tests/utils_/test_network_utils.py -k get_ip_force

python -m pytest -q tests/snapshot/test_model_state.py

Test Result

• Configuration, CLI, and restore utility tests: 6 passed.
• Forced address probing tests: 3 passed.
• Snapshot model-state tests passed.
• Ruff check and format validation passed for the changed Python files.

AI Assistance

This PR was developed with assistance from OpenAI Codex and reviewed by the
author.

---

Essential Elements of an Effective PR Description Checklist

• The purpose of the PR is described and linked to RFC [RFC]: Support VLLM Container Snapshot with Checkpoint/Restore in Userspace (CRIU) #54940.
• The scope and device-plugin boundary are documented.
• The test plan contains the executed commands.
• The test results are included above.
• Documentation update, if requested during RFC review.

BEFORE SUBMITTING, PLEASE READ https://docs.vllm.ai/en/latest/contributing

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[claude]

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[mergify]

This pull request has merge conflicts that must be resolved before it can be
merged. Please rebase the PR, @lbr711.

https://docs.github.com/en/pull-requests/collaborating-with-pull-requests/working-with-forks/syncing-a-fork