refactor: update test fixtures to use Pydantic models

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

tests/conftest.py

@@ -3,6 +3,8 @@
 import pytest
 from pathlib import Path
 
+from agents.design_state import DesignState
+
 
 @pytest.fixture
 def tmp_output_dir(tmp_path):
@@ -25,20 +27,20 @@ def sample_history():
 
 @pytest.fixture
 def empty_design_state():
-    """Empty design state dict."""
-    return {}
+    """Empty design state."""
+    return DesignState()
 
 
 @pytest.fixture
 def populated_design_state():
     """Design state with some decisions already made."""
-    return {
-        "part_name": "servo_bracket",
-        "material": "aluminum 6061",
-        "dimensions": {"width": 60.0},
-        "features": ["4x M4 holes"],
-        "decisions": ["L-bracket form factor"],
-    }
+    return DesignState(
+        part_name="servo_bracket",
+        material="aluminum 6061",
+        dimensions={"width": 60.0},
+        features=["4x M4 holes"],
+        decisions=["L-bracket form factor"],
+    )
 
 
 class FakeLLMBackend: