Refactor duplicated test code into shared utilities

[Copilot]

Identified and consolidated duplicated patterns across the test suite into reusable utilities.

Changes

Created tests/conftest.py with shared utilities:

• electron_energy_MeV_low() / electron_energy_MeV_high() fixtures - replaces duplicate fixtures in test_stopping.py (1000.0 MeV) and test_stopping_models.py (100.0 MeV)
• assert_list_of_type(result, expected_type, expected_first_value=None) - consolidates repetitive validation pattern used in 5 functions across test_materials.py and test_particles.py

Refactored test_cartesian_product.py:

• Extracted generate_test_args(size1, size2, size3) to eliminate duplicated random data generation across 3 test functions

Example

Before:

def test_get_ids():
    ids = pyamtrack.materials.get_ids()
    assert isinstance(ids, list)
    assert len(ids) > 0
    assert all(isinstance(id, int) for id in ids)
    assert ids[0] == 1

After:

def test_get_ids():
    ids = pyamtrack.materials.get_ids()
    assert_list_of_type(ids, int, expected_first_value=1)

Impact: 91 additions, 78 deletions across 6 files; ~32 lines of duplication eliminated

Original prompt

Find and refactor duplicated code

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