Expand unit tests for core classes

[Copilot]

This PR addresses issue #1 by adding comprehensive unit tests for the Ophidian prototype game.

Overview

Created a complete test suite with 73 unit tests covering all major classes in the codebase. The tests are organized in a new tests/ directory and follow pytest conventions, making them compatible with the existing test.sh script.

What's Covered

The test suite provides extensive coverage for:

• Config - All configuration properties including display settings, grid size, tick speed, and game options
• Entity System - Base Entity class with ID management, creation dates, and relationship tracking
• Food & SnakePart - Game entity classes with color management, inheritance verification, and snake part linking
• Location - Entity storage at coordinates, addition/removal operations, and warning messages
• Grid - 2D grid generation, directional navigation (up/down/left/right), entity management, and border handling
• Environment - High-level environment management integrating grids and entities
• Level - Level creation with various sizes and environment initialization

Test Quality

• ✅ All 73 tests pass successfully
• ✅ Tests include normal cases, edge cases, and error conditions
• ✅ Descriptive test names and comprehensive documentation
• ✅ Compatible with existing test.sh coverage script
• ✅ Follows pytest conventions and project code style

Coverage Results

Name                              Stmts   Miss  Cover
-----------------------------------------------------
src/config/config.py                 19      0   100%
src/food/food.py                      7      0   100%
src/level/level.py                    6      0   100%
src/lib/pyenvlib/entity.py           44      0   100%
src/lib/pyenvlib/environment.py      48      2    96%
src/lib/pyenvlib/grid.py             98     13    87%
src/lib/pyenvlib/location.py         34      0   100%
src/snake/snakePart.py               32      0   100%

The test suite achieves 87-100% coverage on core classes, bringing overall project coverage to 48%. The main game loop (ophidian.py) remains untested as it requires pygame integration tests.

Additional Changes

Updated .gitignore to exclude test artifacts:

• .coverage - Coverage data file
• cov.xml - XML coverage report
• .pytest_cache/ - Pytest cache directory

This test suite provides a solid foundation for future development and ensures the core game mechanics work correctly.

Original prompt

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This section details on the original issue you should resolve

<issue_title>Expand unit tests</issue_title>
<issue_description>Create or expand unit tests </issue_description>

Comments on the Issue (you are @copilot in this section)

Fixes #89

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