fix: codebase incompatible with Mesa 4.x (mesa.space removed)

## Description

Mesa 4.x removed `mesa.space` entirely, which breaks the mesa-llm codebase with a `ModuleNotFoundError` on import. Running `python -m pytest tests/` on a fresh Mesa 4.x install fails with 24 test failures.

## Root Cause

The codebase uses the old Mesa 2.x/3.x space API:
- `from mesa.space import MultiGrid, SingleGrid, ContinuousSpace`

These classes no longer exist in Mesa 4.x.

## Fix

Migrate to the new Mesa 4.x APIs:

| Old | New |
|-----|-----|
| `mesa.space.MultiGrid` | `mesa.discrete_space.OrthogonalMooreGrid` |
| `mesa.space.SingleGrid` | `mesa.discrete_space.OrthogonalMooreGrid` |
| `mesa.space.ContinuousSpace` | `mesa.experimental.continuous_space.ContinuousSpace` |
| `grid.place_agent(agent, pos)` | `agent.cell = grid._cells[pos]` |
| `grid.get_neighbors()` | `cell.get_neighborhood(radius=vision)` |
| `model.steps` | `model.step` |

## Result

All 203 tests passing after migration.

## Files Affected
- `mesa_llm/llm_agent.py`
- `mesa_llm/tools/inbuilt_tools.py`
- `mesa_llm/recording/record_model.py`
- `tests/conftest.py`
- `tests/test_llm_agent.py`
- `tests/test_parallel_stepping.py`
- `tests/test_reasoning/test_cot.py`
- `tests/test_tools/test_inbuilt_tools.py`

Start with the Mesa imports and grid usage in mesa_llm/llm_agent.py, mesa_llm/tools/inbuilt_tools.py, and mesa_llm/recording/record_model.py, then review the related fixtures and tests listed in the issue. Run python -m pytest tests/ before and after the migration. Done means the code uses Mesa 4.x APIs and all 203 tests pass.

Written by the indexing model from the issue text.