fix: preserve sample_query_options during cohort normalization

[kshirajahere]

Summary

Before this change, sample_query_options could still be lost inside shared cohort normalisation even after the recent caller-level forwarding fixes. Any multi-cohort path that rebuilt cohort queries and then rechecked cohort sizes would fail for valid parameterised pandas queries such as country in @countries_list.

After this change, cohort normalisation preserves the full query context, and the shared query helpers now treat engine="python" as a default rather than forcing it as a duplicate keyword.

Closes #1100.

Why this matters

This is a framework-level correctness fix, not just a surface patch:

• pairwise_average_fst() was still broken for local_dict-backed filters because _setup_cohort_queries() re-applied the combined cohort query without sample_query_options.
• Multi-cohort H12 plotting had the same underlying problem for variable-backed sample filters.
• Once query options are forwarded correctly, sample_metadata() and _filter_sample_dataset() must also accept an explicit engine option without raising TypeError.

The net effect is that the public sample_query_options contract now behaves consistently across direct metadata selection and higher-level cohort analyses.

Exact changes

• add _prep_sample_query_options() in AnophelesBase to preserve user-supplied query context while defaulting the engine to python
• use that helper in shared query-evaluation paths instead of passing engine="python" as a duplicate keyword
• pass sample_query_options through _setup_cohort_queries() when validating derived cohort queries
• add regressions for:
  • direct sample_metadata() queries using both engine and local_dict
  • pairwise_average_fst() with local_dict-backed sample_query
  • multi-cohort H12 plotting with local_dict-backed sample_query

Tests run

• ruff check malariagen_data/anoph/base.py malariagen_data/anoph/sample_metadata.py tests/anoph/test_fst.py tests/anoph/test_h12.py tests/anoph/test_sample_metadata.py
• pytest tests/anoph/test_h12.py tests/anoph/test_fst.py -q
• pytest tests/anoph/test_sample_metadata.py -k 'sample_metadata_with_query or sample_metadata_with_indices or query_options' -q