feat: make contexts optional in /api/score

When contexts is absent, metrics that require retrieved_contexts
(faithfulness, context_recall, context_precision, noise_sensitivity)
are automatically skipped and appear in skipped_metrics.
Only answer_relevancy, factual_correctness, semantic_similarity
remain computable without contexts.

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

tests/webapp/test_score_api.py

@@ -57,9 +57,11 @@ class TestScoreRequest:
         with pytest.raises(ValidationError):
             ScoreRequest(question="q", contexts="c")  # type: ignore[call-arg]
 
-    def test_missing_contexts_raises(self):
-        with pytest.raises(ValidationError):
-            ScoreRequest(question="q", answer="a")  # type: ignore[call-arg]
+    def test_missing_contexts_defaults_to_none(self):
+        """contexts is now optional — missing contexts is allowed."""
+        req = ScoreRequest(question="q", answer="a")
+        assert req.contexts is None
+        assert req.contexts_as_list() == []
 
     def test_custom_metrics_accepted(self):
         req = ScoreRequest(
@@ -115,6 +117,17 @@ class TestScoreRequest:
             "factual_correctness",
         ]
 
+    def test_effective_metrics_drops_context_dependent_when_contexts_absent(self):
+        """Without contexts, context-dependent metrics are excluded."""
+        req = ScoreRequest(
+            question="q", answer="a",
+            metrics=["faithfulness", "answer_relevancy", "context_precision"],
+        )
+        effective = req.effective_metrics()
+        assert "answer_relevancy" in effective
+        assert "faithfulness" not in effective
+        assert "context_precision" not in effective
+
 
 class TestScoreResponse:
     def test_score_response_structure(self):