fix(settings): use explicit None/empty guard for Path serializer

`Path` has no `__bool__`, making all instances truthy. The previous
`if not input_value:` guard was therefore never entered for any `Path`,
so `Path("")` silently resolved to the current working directory.

Replace the falsy check with an explicit guard:
  if input_value is None or not input_value.parts:

This correctly returns `None` for both `None` and empty/no-parts paths
(e.g. `Path("")` / `Path()`), while resolving all other paths as before.

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

Author: olivermeyer

src/aignostics_foundry_core/settings.py

@@ -63,9 +63,10 @@ def serialize_path_resolve(input_value: Path | None, _info: FieldSerializationIn
             _info: Pydantic serialization info (unused).
 
         Returns:
-            None if input is falsy, otherwise the resolved absolute path string.
+            None if input is `None` or has no path components (e.g. empty string),
+            otherwise the resolved absolute path string.
         """
-        if not input_value:
+        if input_value is None or not input_value.parts:
             return None
         return str(input_value.resolve())
 

tests/aignostics_foundry_core/settings_test.py

@@ -117,6 +117,12 @@ def test_serialize_path_resolve_none(self) -> None:
         result = OpaqueSettings.serialize_path_resolve(cast("Path", None), _make_info(None))
         assert result is None
 
+    @pytest.mark.unit
+    def test_serialize_path_resolve_empty_path(self) -> None:
+        """Test that None is returned for Path("") rather than resolving to CWD."""
+        result = OpaqueSettings.serialize_path_resolve(Path(), _make_info(None))
+        assert result is None
+
 
 class TestLoadSettings:
     """Tests for load_settings."""