fix: terminate detached descendants on bbdev cancellation

[daiyongyuan]

Summary

• Terminate the complete subprocess tree when a bbdev task is cancelled, shut down, or times out.
• Clean up descendants that start a separate session, such as detached FPGA flashing tools.

Validation

• api/.venv/bin/python -m unittest api.unit_tests.test_process_cleanup -v (2 tests passed)
• git diff --check

[shirohasuki]

Tests under api/tests are for bbdev API testing and demonste how to use them; please read the unified format in other tests.