fix: make ResearchReport.references optional with default

The LLM doesn't always return the references field, causing validation
failures. Adding default_factory=list makes it optional.

TDD: Added tests/unit/utils/test_models.py with coverage for this case.

Fixes #23

src/utils/models.py

@@ -213,7 +213,8 @@ class ResearchReport(BaseModel):
     conclusion: str = Field(description="Overall conclusion")
 
     references: list[dict[str, str]] = Field(
-        description="Formatted references with title, authors, source, URL"
+        default_factory=list,
+        description="Formatted references with title, authors, source, URL",
     )
 
     # Metadata

tests/unit/utils/test_models.py

@@ -0,0 +1,54 @@
+"""Unit tests for data models."""
+
+from src.utils.models import ReportSection, ResearchReport
+
+
+class TestResearchReport:
+    """Tests for ResearchReport model."""
+
+    def test_references_has_default(self) -> None:
+        """ResearchReport should allow creation without references field.
+
+        This is critical because LLMs don't always return the references
+        field, and we shouldn't fail validation when it's missing.
+        """
+        report = ResearchReport(
+            title="Test Report",
+            executive_summary="A" * 100,  # min_length=100
+            research_question="Does drug X help condition Y?",
+            methodology=ReportSection(title="Methods", content="We searched..."),
+            hypotheses_tested=[{"hypothesis": "test", "supported": True}],
+            mechanistic_findings=ReportSection(title="Mechanisms", content="Found..."),
+            clinical_findings=ReportSection(title="Clinical", content="Trials show..."),
+            drug_candidates=["Drug A"],
+            limitations=["Small sample"],
+            conclusion="Promising results.",
+            sources_searched=["pubmed"],
+            total_papers_reviewed=10,
+            search_iterations=2,
+            confidence_score=0.8,
+            # NOTE: references intentionally omitted
+        )
+
+        # Should have empty list as default
+        assert report.references == []
+
+    def test_references_can_be_provided(self) -> None:
+        """ResearchReport should accept references when provided."""
+        refs = [{"title": "Paper 1", "url": "https://example.com"}]
+        report = ResearchReport(
+            title="Test Report",
+            executive_summary="A" * 100,
+            research_question="Does drug X help condition Y?",
+            methodology=ReportSection(title="Methods", content="We searched..."),
+            hypotheses_tested=[],
+            mechanistic_findings=ReportSection(title="Mechanisms", content="Found..."),
+            clinical_findings=ReportSection(title="Clinical", content="Trials show..."),
+            drug_candidates=[],
+            limitations=[],
+            conclusion="Results.",
+            confidence_score=0.5,
+            references=refs,
+        )
+
+        assert report.references == refs