Expand Unit Test Coverage

[Technical-1]

Priority: Medium
Labels: testing, code-quality

Description:
Add tests for error conditions (poisoned mutex, invalid PIDs), edge cases (empty process list, zero memory), and concurrent access patterns.

Files to modify:

• tests/test_system.rs
• tests/test_concurrent.rs (new file)

Acceptance criteria:

• Tests for error conditions added
• Tests for edge cases added
• Tests for concurrent access patterns added
• Test coverage increased significantly
• All tests pass consistently

Implementation notes:

• Add tests for mutex poisoning scenarios
• Add tests for invalid PID lookups
• Add tests for empty process lists
• Add tests for zero memory scenarios
• Add concurrent access tests using multiple threads