fix: handle explicit none extra headers

Author: eric-tramel

packages/data-designer-engine/src/data_designer/engine/models/facade.py

@@ -158,11 +158,11 @@ def consolidate_kwargs(self, **kwargs: Any) -> dict[str, Any]:
         if self.model_provider.extra_body:
             kwargs["extra_body"] = {**kwargs.get("extra_body", {}), **self.model_provider.extra_body}
         if self.model_provider.extra_headers:
-            kwargs["extra_headers"] = {**kwargs.get("extra_headers", {}), **self.model_provider.extra_headers}
+            kwargs["extra_headers"] = {**(kwargs.get("extra_headers") or {}), **self.model_provider.extra_headers}
         # Inject framework-level attribution header when telemetry is enabled.
         # Applied last so that user-supplied or provider-level headers take precedence.
         if TELEMETRY_ENABLED:
-            headers = kwargs.get("extra_headers", {})
+            headers = kwargs.get("extra_headers") or {}
             if "X-Title" not in headers:
                 kwargs["extra_headers"] = {"X-Title": ATTRIBUTION_TITLE, **headers}
         return kwargs

packages/data-designer-engine/tests/engine/models/test_facade.py

@@ -252,11 +252,24 @@ def test_consolidate_kwargs_telemetry_disabled(stub_model_configs: list[Any], st
 def test_consolidate_kwargs_user_x_title_override(
     stub_model_configs: list[Any], stub_model_facade: ModelFacade
 ) -> None:
-    """User-supplied X-Title via provider extra_headers takes precedence over framework default."""
+    """User-supplied X-Title takes precedence over the framework default."""
     stub_model_facade.model_provider.extra_headers = {"X-Title": "My Custom App"}
     result = stub_model_facade.consolidate_kwargs()
     assert result["extra_headers"]["X-Title"] == "My Custom App"
 
+    stub_model_facade.model_provider.extra_headers = None
+    result = stub_model_facade.consolidate_kwargs(extra_headers={"X-Title": "Caller App"})
+    assert result["extra_headers"]["X-Title"] == "Caller App"
+
+
+def test_consolidate_kwargs_with_explicit_none_extra_headers(
+    stub_model_configs: list[Any], stub_model_facade: ModelFacade
+) -> None:
+    """Explicit None extra_headers does not break provider merges or framework attribution."""
+    stub_model_facade.model_provider.extra_headers = {"hello": "world"}
+    result = stub_model_facade.consolidate_kwargs(extra_headers=None)
+    assert result["extra_headers"] == {"X-Title": "NeMo Data Designer", "hello": "world"}
+
 
 @pytest.mark.parametrize(
     "skip_usage_tracking",