Fix MR subvariable in filters

[slobodan-ilic]

Summary

• avoid whole-MR expansion when an expression is already scoped to a subvariable via axes
• add regression coverage for both subvar1 in [1] and MyMrVar[subvar1] in [1]

Diagnosis

The 0.18.24 MR rewrite treats list-valued filters on MR variables as whole-variable filters that need expansion across all subvariables. If the variable term has already been rewritten to {var: parent, axes: [subvar]}, there is no array list to update, so _update_values_for_multiple_response() crashes on arrays[0] with IndexError: list assignment index out of range.

Tests

• venv/bin/python -m pytest scrunch/tests/test_expressions.py::TestExpressionProcessing::test_multiple_response_subvar_in scrunch/tests/test_expressions.py::TestExpressionProcessing::test_multiple_response_subvar_equality scrunch/tests/test_expressions.py::TestExpressionProcessing::test_process_in_multiple_response_without_entity_key scrunch/tests/test_expressions.py::TestExpressionProcessing::test_process_in_multiple_response scrunch/tests/test_expressions.py::TestExpressionProcessing::test_process_all_multiple_response scrunch/tests/test_expressions.py::TestExpressionProcessing::test_multiple_response_any_process_single_subvariables scrunch/tests/test_expressions.py::TestExpressionProcessing::test_multiple_response_any_process_two_subvariables -q
• venv/bin/python -m pytest scrunch/tests -q