🧪 Add unit tests for safe_cell_removal in CancerInvasionSteppables

[NoWon1]

🎯 What: Added tests for the safe_cell_removal method in Simulation/CancerInvasionSteppables.py to cover both inner and outer exception handling blocks.
📊 Coverage: Covered the happy path (cell removal successful), missing cell path, inner try/except block (Exception when accessing an individual cell coordinate), and outer try/except block (Exception when determining the dimensions).
✨ Result: Improved test coverage and reliability of the codebase. Tested without mutating global mocks in unittest.mock. Cleaned up unnecessary artifacts like __pycache__.

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PR created automatically by Jules for task 16694932118922055440 started by @NoWon1

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