Fix missing volume checks in material activity/decay heat

[approx-infinity]

Summary

This PR adds explicit validation for Material.volume in total activity (Bq, Ci) and decay heat (W) calculations.

Previously, calling get_activity() or get_decay_heat() without setting a material volume could result in unclear or misleading runtime errors (e.g., TypeError or downstream failures).

This change introduces a clear and early ValueError when volume is not defined, improving error clarity and robustness of the material depletion/decay interface.

Changes

• Added explicit check for Material.volume before computing:
  -- total activity
  -- total decay heat
• Raise ValueError with a descriptive message if volume is not set
• Ensures failure occurs at the correct abstraction level (input validation instead of downstream numerical failure)

Testing

pytest tests/unit_tests/test_material.py -k "get_activity or get_decay_heat"

Result

2 passed, 35 deselected

Checklist

• I have performed a self-review of my own code
• I have run clang-format (version 18) on any C++ source files (if applicable)
• I have followed the style guidelines for Python source files (if applicable)
• I have made corresponding changes to the documentation (if applicable)
• I have added tests that prove my fix is effective or that my feature works (if applicable)

[nuclearkevin]

Thanks for this contribution, and welcome to the OpenMC community @approx-infinity! This looks good to me; if nobody else has any comments I'll merge tomorrow.