Fixes #3105

This PR improves Cell.get_neighborhood() by replacing the recursive implementation with an iterative Breadth-First Search (BFS). This change provides two key benefits:

1. Performance improvement: ~2x faster neighborhood lookups for all radius values
2. Eliminates edge-case crashes: Prevents RecursionError when radius exceeds Python's recursion limit (~1000)

Problem

The previous implementation used recursion to traverse cell neighborhoods:

for neighbor in self.connections.values():
    neighborhood.update(
        neighbor._neighborhood(radius - 1, include_center=True)  # Recursive call
    )

This had two issues:

• Performance: Function call overhead from deep recursion slowed down neighborhood queries
• Edge-case crashes: When radius > sys.getrecursionlimit() (~1000), it caused RecursionError: maximum recursion depth exceeded

Solution

Replaced with iterative BFS using explicit layer-by-layer traversal:

# Use iterative BFS for better performance and no recursion limit
visited: set[Cell] = {self}
current_layer: list[Cell] = list(self.connections.values())
neighborhood: dict[Cell, list[Agent]] = {}

# Add immediate neighbors (radius=1)
for neighbor in current_layer:
    if neighbor not in visited:
        visited.add(neighbor)
        neighborhood[neighbor] = neighbor._agents

# Expand outward for remaining radius levels
for _ in range(radius - 1):
    next_layer: list[Cell] = []
    for cell in current_layer:
        for neighbor in cell.connections.values():
            if neighbor not in visited:
                visited.add(neighbor)
                next_layer.append(neighbor)
                neighborhood[neighbor] = neighbor._agents
    current_layer = next_layer
    if not current_layer:
        break

Benefits

~2x faster neighborhood lookups (confirmed by maintainer benchmarks)
No recursion limit: Can handle any radius value (memory-limited, not stack-limited)
Same behavior: All 37 existing discrete_space tests pass
Same time complexity: O(number of cells in radius)

Changes

• mesa/discrete_space/cell.py: Replaced recursive _neighborhood() with iterative BFS
• tests/discrete_space/test_discrete_space.py: Added 3 regression tests:
  • test_large_radius_neighborhood: Tests radius=1500 on a 2000-cell chain
  • test_large_radius_with_include_center: Tests include_center parameter with large radius
  • test_radius_exceeds_reachable_cells: Tests radius > available cells

Testing

$ python -m pytest tests/discrete_space/test_discrete_space.py -v
============================= 37 passed in 13.67s =============================

All existing tests pass + 3 new regression tests added.