test: stabilize queue-owner integration teardown

[dutifulbob]

Problem

CI failed intermittently in this job:
https://github.com/openclaw/acpx/actions/runs/22529835998/job/65267489405

Failing test:

• integration: cancel yields cancelled stopReason without queue error

Observed failure:

• ENOTEMPTY: directory not empty, rmdir ~/.acpx/sessions

Root cause: detached queue-owner processes could still be active while integration temp-home cleanup ran, causing teardown races.

Changes

• Added explicit session shutdown in integration tests that start detached queue owners.
• Added waits for queue-owner PID exit before temp-home cleanup where needed.
• Added defensive cleanup in the cancel integration test (stop child -> close session -> wait for owner exit).

Scope is test-only; no production/runtime code changes.

Validation

• npm run lint
• npm test
• Reproduced target test repeatedly:
  • npm run build:test && for i in $(seq 1 20); do node --test --test-name-pattern "integration: cancel yields cancelled stopReason without queue error" dist-test/test/integration.test.js || exit 1; done
  • Result: 20/20 passes locally.