test: add cases to increase coverage for cpu.py

[sql-hkr]

Add additional test cases to improve coverage, particularly for cpu.py.

Purpose

• Increase Codecov coverage for cpu.py
• Ensure edge cases and all major logic paths are tested
• Maintain test suite clarity and simplicity

Tasks

• Identify untested functions or branches in cpu.py
• Write additional unit tests for these cases
• Verify coverage increase in Codecov
• Ensure tests run efficiently