fix(v1): avoid false shutdown failures on clean exit

[ShuhaoZhangTony]

Problem

A clean or externally initiated V1 engine-core exit could be logged as an unexpected process death. The client could also leave background resources alive, and the legacy synchronous engine did not expose an explicit cleanup path for all owned resources.

Fix

• Mark explicit MPClient shutdowns as intentional before stopping child processes.
• On a clean manager exit, shut down client sockets and background tasks without logging a false crash.
• Preserve the unexpected-death error path when a failed process name is present.
• Add an idempotent LLMEngine.shutdown() path for Prometheus state, optional renderer cleanup, engine core, and owned DP groups.

Validation

• Added focused regression tests for explicit client shutdown, clean monitor exit, renderer cleanup, renderer implementations without shutdown, engine-core cleanup, and DP-group teardown.
• ruff check vllm/v1/engine/core_client.py vllm/v1/engine/llm_engine.py tests/v1/engine/test_shutdown_cleanup.py
• ruff format --check vllm/v1/engine/core_client.py vllm/v1/engine/llm_engine.py tests/v1/engine/test_shutdown_cleanup.py
• python3 -m compileall -q vllm/v1/engine/core_client.py vllm/v1/engine/llm_engine.py tests/v1/engine/test_shutdown_cleanup.py

This is a shutdown-correctness fix and makes no performance claim.\n\n### Current-head validation (2026-07-26)\n\n- Rebased onto upstream main 0ba2aa35a81dcc3246b26291368b53fa2389c7d7.\n- Exact PR head: fca28675723dc7eff3ec21c07ff9510b3b0bf9c0.\n- Focused shutdown suite passed in the CANN 9.0.0 container: 4 passed.\n- Ruff check, Ruff format check, and git diff --check passed.\n\n\n## Contribution disclosure

• Duplicate-work check: an open-PR search for v1 clean shutdown false failure found no competing implementation; the unrelated matches address sampling, CRIU snapshots, and a kernel alignment fix.
• AI assistance was used for implementation support, test execution, rebase maintenance, and drafting this description. The human submitter remains responsible for reviewing and defending the change.

[claude]

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[ShuhaoZhangTony]

Gentle ping for maintainer review. The current pre-run-check failure is the contribution gate rather than a code/test failure: this author needs a maintainer-applied verified, ready, or ready-run-all-tests label. Could a maintainer please review and add the appropriate label if the change is acceptable?