Summary

This PR introduces a minimal, safe, and non-breaking Mesa-specific exception hierarchy.
It defines a base exception (MesaError) and three domain-specific subclasses:

• ModelError – for errors related to model configuration, initialization, or execution
• AgentError – for errors related to agent lifecycle or behavior
• SpaceError – for errors related to spaces, grids, movement, or spatial constraints

This provides a foundation for future exception standardization and migration of existing generic exceptions.

Motivation

• Issue Mesa 4 development tracking issue #3132 tracks Mesa 4 development and highlights the need for a dedicated exception hierarchy.
• A previous attempt (closed PR Feat: Introduce dedicated exception hierarchy in mesa/errors.py  #2992) proposed a full hierarchy, but it was too large and unmergeable at the time.
• This PR intentionally takes a minimal and incremental approach, allowing the hierarchy to be adopted gradually.

Key Features

• Uses from __future__ import annotations to avoid circular import issues
• Defines __all__ for a clean, explicit public API
• Safely stores Mesa version using importlib.metadata without importing mesa
• Empty domain-specific subclasses include pass for linter compliance
• Non-breaking: does not change any existing behavior or messages
• Ready for future migration of existing exceptions in Space, Model, or Agent modules

Example Usage

from mesa.errors import MesaError, SpaceError

# Raising a domain-specific exception
if not self.is_cell_empty(pos):
    raise SpaceError(f"Cell {pos} is already occupied")

# Catching all Mesa exceptions safely
try:
    model.step()
except MesaError as e:
    print(f"Mesa error occurred (v{e.mesa_version}): {e}")

• 🛠 Feature/enhancement