fix: avoid gevent signal shutdown error

[suguanYang]

Summary

• remove REMAP_SIGTERM=SIGQUIT from the staging worker task definition
• keep ECS stopTimeout=120 and Celery soft-shutdown settings
• update the worker shutdown contract to require the normal SIGTERM path

Root cause

ECS sends SIGTERM. The remap routed it into Celery 5.5.3's cold-shutdown handler, which calls its soft-shutdown sleep from a gevent signal/event-loop callback. The staging drill reproduced gevent.exceptions.BlockingSwitchOutError.

The normal SIGTERM warm-shutdown path avoids that incompatible callback while retaining late acknowledgements and ECS task replacement behavior.

Verification

• uv run --directory /home/suguan/github.com/ontosAI/knowhere pytest -q apps/worker/tests/contract/test_worker_shutdown_contract.py apps/worker/tests/contract/test_worker_bootstrap_contract.py
• Result: 3 passed

Staging deployment and interruption re-test are intentionally deferred until this PR is reviewed and merged.