Add tests for NullHyperparameterLikelihood variance and threshold behavior

[Copilot]

The NullHyperparameterLikelihood class lacked test coverage for its core behaviors: variance calculation consistency with the parent class and threshold enforcement for rejecting high-variance samples.

Changes

Added NullHyperparameterLikelihoodTests test class with three tests:

• test_null_likelihood_variance_matches_hyperparameter_likelihood: Verifies variance calculation matches parent HyperparameterLikelihood exactly
• test_null_likelihood_returns_zero: Confirms likelihood always returns 0.0 regardless of parameters
• test_null_likelihood_returns_neginf_when_variance_exceeds_threshold: Validates that log_likelihood_ratio() returns -inf when variance exceeds maximum_uncertainty threshold

# Example: Variance matching test
null_like = NullHyperparameterLikelihood(posteriors=data, hyper_prior=model)
hyper_like = HyperparameterLikelihood(posteriors=data, hyper_prior=model)

null_ln_l, null_variance = null_like.ln_likelihood_and_variance(params)
hyper_ln_l, hyper_variance = hyper_like.ln_likelihood_and_variance(params)

assert null_ln_l == 0.0  # Null likelihood always zero
assert null_variance == hyper_variance  # But variance matches parent

Tests follow existing patterns in likelihood_test.py and integrate with the existing 22-test suite.

Original prompt

Add a test for the NullHyperparmeterLikelihood testing that the variance matches the HyperparameterLikelihood and returns zero for the likelihood when the variance is less than the threshold.

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[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 88.64%. Comparing base (ba7ab69) to head (f339226).
⚠️ Report is 1 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #133      +/-   ##
==========================================
+ Coverage   88.42%   88.64%   +0.22%     
==========================================
  Files          14       14              
  Lines         907      907              
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+ Hits          802      804       +2     
+ Misses        105      103       -2     

Flag	Coverage Δ
python3.10	88.64% <ø> (+0.22%)	⬆️
python3.11	88.64% <ø> (+0.22%)	⬆️
python3.12	88.64% <ø> (+0.22%)	⬆️
python3.13	88.64% <ø> (+0.22%)	⬆️

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