fix: migrate codebase to Mesa 4.x API (resolves mesa.space removal)

[abhinavk0220]

Closes #152

Summary

Mesa 4.x removed mesa.space entirely. This PR migrates the full codebase to the new Mesa 4.x APIs so that mesa-llm works with the current version of Mesa.

Changes

• Replace MultiGrid/SingleGrid with OrthogonalMooreGrid from mesa.discrete_space
• Replace old ContinuousSpace(x_max, y_max) with new dimensions-based API
• Fix agent placement to use cell-based API (agent.cell = grid._cells[pos])
• Fix neighbor lookup using get_neighborhood() + coordinate set matching
• Fix model.steps → model.step (renamed in Mesa 4.x)
• Fix Model.__init__(seed=...) kwarg incompatibility with mesa_signals

Testing

All 203 tests passing after migration:

Notes

Tested against Mesa 4.x dev installed from source.

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[Jay-Lokhande]

Hey this looks cool,
actually i have cloned your branch but not getting how did you tested against Mesa 4.x would please help me out for this?

[colinfrisch]

I'm not fully up to date with the latest mesa 4.x changes... @wang-boyu what do you think about this ?

[abhinavk0220]

Hey Jay! To test against Mesa 4.x, install Mesa directly from
source:

git clone https://github.com/mesa/mesa.git
cd mesa
pip install -e .

Then install mesa-llm dependencies:

cd ../mesa-llm
pip install -e .

Then run the test suite:

python -m pytest tests/ -q

The key Mesa 4.x changes that affect mesa-llm are:

• mesa.space is removed → use mesa.discrete_space and
  mesa.experimental.continuous_space
• Model(seed=42) constructor changed
• model._time replaces model.steps as step counter

Happy to help if you run into issues! 🙂

[abhinavk0220]

Hi Colin! Happy to provide more context for wang-boyu's review.

The key Mesa 4.x breaking changes this PR addresses:

1. mesa.space removed entirely → migrated to mesa.discrete_space
   (OrthogonalMooreGrid) and mesa.experimental.continuous_space

2. ContinuousSpace API changed → old positional args replaced
   with dimensions=[[0,x],[0,y]] keyword API

3. model.steps → model._time as the internal step counter

4. Model(seed=42) constructor incompatibility with mesa_signals

All 203 tests passing after migration. Happy to answer any
specific questions wang-boyu has!