541 missing average energy for mixture node

[abpolym]

Should resolve #541

The Mixture node performs model comparison via scale factors, so its contribution to the Bethe Free Energy (BFE) is not well-defined.
To avoid runtime errors and make BFE computations robust, we now define a placeholder @average_energy method for Mixture that:

• emits a clear warning,
• returns 0.0 as a safe default.

This ensures that global BFE evaluation does not fail when mixtures are present, while still alerting users that the term is undefined conceptually.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 76.17%. Comparing base (8a0be85) to head (1127f62).
⚠️ Report is 9 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #546   +/-   ##
=======================================
  Coverage   76.17%   76.17%           
=======================================
  Files         207      207           
  Lines        6149     6149           
=======================================
  Hits         4684     4684           
  Misses       1465     1465           

☔ View full report in Codecov by Sentry.
📢 Have feedback on the report? Share it here.

🚀 New features to boost your workflow:

• ❄️ Test Analytics: Detect flaky tests, report on failures, and find test suite problems.