🧪 [Testing Improvement] Add unit tests for create_simple_fiber

[NoWon1]

🎯 What: This PR introduces unit tests for the create_simple_fiber geometry generation function in Simulation/CancerInvasionSteppables.py. The simulation's dependency on cc3d has been properly mocked to allow for fully isolated testing in standard Python environments. Also added .gitignore to prevent tracking of bytecode files.

📊 Coverage:

• Happy Path: Verifies correct linear pixel sequence generation.
• Boundary Conditions: Validates that coordinates outside the bounds [50, 450) are strictly ignored.
• Error Handling: Ensures that invalid types passed to the function are caught, safely handled, and result in returning an empty list rather than crashing.

✨ Result: Enhanced test coverage that catches breaking changes in the geometry generator, leading to more resilient simulations.

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PR created automatically by Jules for task 5527358725954347933 started by @NoWon1

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