fix: address code review issues from PR #358 (#355)

- rdf_exporter.py: add isinstance(format, str) guard before .lower() so
  non-string inputs (None, int, etc.) raise ValidationError consistently
  instead of AttributeError; normalize via strip().lower() in one step
- 15_Export.ipynb: fix notebook cell using result['valid'] → result['overall_valid']
  (validate_rdf() returns overall_valid, not valid); add trailing EOF newline
- test_rdf_exporter.py: add tests for non-string format → ValidationError
  and for overall_valid key presence in validate_rdf() return value

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

tests/export/test_rdf_exporter.py

@@ -71,3 +71,21 @@ def test_ttl_export_to_file(exporter, tmp_path):
     exporter.export(RDF_DATA, str(out), format="ttl")
     assert out.exists()
     assert out.stat().st_size > 0
+
+
+def test_non_string_format_raises_validation_error(exporter):
+    """format=None or non-string must raise ValidationError, not AttributeError."""
+    from semantica.utils.exceptions import ValidationError
+
+    with pytest.raises(ValidationError):
+        exporter.export_to_rdf(RDF_DATA, format=None)
+
+    with pytest.raises(ValidationError):
+        exporter.export_to_rdf(RDF_DATA, format=123)
+
+
+def test_validate_rdf_returns_overall_valid_key(exporter):
+    """validate_rdf() must return 'overall_valid' key (used in notebook example)."""
+    result = exporter.validate_rdf(RDF_DATA)
+    assert "overall_valid" in result
+    assert isinstance(result["overall_valid"], bool)