Fix a bug in Cell.add_agent() where cells with capacity=0 incorrectly allowed unlimited agents instead of rejecting all agents.

Problem

The condition if self.capacity and n >= self.capacity: treated capacity=0 as falsy, which skipped the capacity check entirely. This meant cells with zero capacity could hold unlimited agents, which is incorrect behavior.

Example of the bug:

cell = Cell(coordinate=(0, 0), capacity=0)
agent = CellAgent(model)
cell.add_agent(agent)  # Should raise exception, but didn't
print(len(cell.agents))  # Output: 1 (should be 0)

Solution

Changed the condition from if self.capacity to if self.capacity is not None to properly distinguish between:

• capacity=None → unlimited agents allowed
• capacity=0 → no agents allowed
• capacity=1, 2, 3... → limited by the specified capacity

Changes Made

• Fixed the capacity check logic in mesa/discrete_space/cell.py
• Added test case for capacity=0 in tests/test_discrete_space.py

Testing

✓ Existing capacity tests still pass
✓ New test verifies capacity=0 raises exception when adding agents
✓ All cell-related tests pass

Impact

This fix addresses a logical bug in the capacity checking mechanism. Please review:

• Code that may have relied on the previous capacity=0 behavior (which allowed unlimited agents) could be affected. This is considered fixing unintended behavior rather than a regression.
• No expected impact on code using capacity=None (unlimited) or positive capacity values.

Reviewers are welcome to discuss whether this change aligns with the intended design and if any deprecation or migration guidance is needed.