Improve test coverage for import_threaded.py and importer.py

- Added comprehensive tests to improve coverage of import_threaded.py from ~53% to 79%
- Added targeted tests to improve coverage of importer.py from ~73% to 80%
- Fixed mypy type annotation issues in new test files
- Fixed ruff unused variable warnings (prefixed with _)
- All tests pass (564/564)
- Project coverage now at 88.70% (above 85% target)
- Mypy sessions now pass across all Python versions (3.9-3.13)

Author: bosd

tests/test_import_threaded_final_coverage.py

@@ -13,6 +13,7 @@
 def test_read_data_file_all_fallbacks_fail() -> None:
     """Test _read_data_file when all fallback encodings fail."""
     with patch("builtins.open") as mock_open:
+
         def side_effect(filename: str, encoding: str, **kwargs: Any) -> None:
             # Always raise UnicodeDecodeError regardless of encoding tried
             raise UnicodeDecodeError("utf-8", b"test", 0, 1, "fake error")
@@ -43,7 +44,7 @@ def test_safe_convert_field_value_id_suffix() -> None:
 def test_handle_create_error_tuple_index_out_of_range() -> None:
     """Test _handle_create_error with tuple index out of range."""
     error = Exception("tuple index out of range")
-    error_str, failed_line, summary = _handle_create_error(
+    error_str, _failed_line, _summary = _handle_create_error(
         0, error, ["test", "data"], "original summary"
     )
     assert "Tuple unpacking error" in error_str

tests/test_importer_coverage.py

@@ -15,7 +15,7 @@ def test_count_lines_with_content() -> None:
     """Test _count_lines with actual content."""
     import tempfile
 
-    with tempfile.NamedTemporaryFile(mode='w', delete=False, suffix='.csv') as f:
+    with tempfile.NamedTemporaryFile(mode="w", delete=False, suffix=".csv") as f:
         f.write("line1\nline2\nline3\n")
         temp_path = f.name