fix: prevent div-by-zero in evaluator when base_refusals is 0 (#225)

* fix: prevent div-by-zero in evaluator when base_refusals is 0

When a model refuses all prompts from the start, base_refusals is 0.
Return refusals directly in that case so ablations that introduce new
refusals are still penalized correctly.

* fix: cast refusals to float for type consistency" before hitting commit changes

Co-authored-by: gemini-code-assist[bot] <176961590+gemini-code-assist[bot]@users.noreply.github.com>

---------

Co-authored-by: gemini-code-assist[bot] <176961590+gemini-code-assist[bot]@users.noreply.github.com>

Author: cpagac

src/heretic/evaluator.py

@@ -110,7 +110,9 @@ def get_score(self) -> tuple[tuple[float, float], float, int]:
         kl_divergence_scale = self.settings.kl_divergence_scale
         kl_divergence_target = self.settings.kl_divergence_target
 
-        refusals_score = refusals / self.base_refusals
+        refusals_score = (
+            refusals / self.base_refusals if self.base_refusals > 0 else float(refusals)
+        )
 
         if kl_divergence >= kl_divergence_target:
             kld_score = kl_divergence / kl_divergence_scale