Fix refusal detector missing 'not able to' and 'outside scope' phrases

Responses like "I'm not able to answer ... falls outside the knowledge base"
were scoring faithfulness 0/1 instead of auto-passing. Added two new branches
to _REFUSAL_PATTERNS to cover these phrasings.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

backend/grader.py

@@ -90,7 +90,9 @@ RELEVANCY_THRESHOLD = 0.45
 FAITHFULNESS_THRESHOLD = 0.35
 
 _REFUSAL_PATTERNS = re.compile(
-    r"(i (don't|do not|cannot|can't) (have|find|provide|answer)|"
+    r"(i (don't|do not|cannot|can't|'m not able to) (have|find|provide|answer)|"
+    r"not able to (answer|provide|help)|"
+    r"(falls?|is) outside (of )?(the )?(scope|knowledge base)|"
     r"not enough (information|context)|"
     r"the (context|provided) (does not|doesn't) (contain|include|mention))",
     re.IGNORECASE,

tests/unit/test_grader.py

@@ -230,6 +230,13 @@ class TestGradeFaithfulnessDecomposed:
         assert result.passed is True
         assert result.score == 1.0
 
+    def test_refusal_not_able_to_auto_passes(self) -> None:
+        result = grade_faithfulness_decomposed(
+            "I'm not able to answer that as it falls outside the knowledge base.", CONTEXT
+        )
+        assert result.passed is True
+        assert result.score == 1.0
+
     def test_empty_context_fails(self) -> None:
         with patch("grader.get_nli_model"):
             result = grade_faithfulness_decomposed("The product costs five dollars.", "")