Remove SQLModel query deprecation warnings

[MaxGhenis]

Summary

• replace legacy session.query(...) lookups with session.exec(select(...)) in the SOI ETL loaders
• update the focused national-target test to use the same SQLModel pattern and avoid test-side warnings
• add a changelog fragment for the CI warning cleanup

Testing

• UV_PYTHON=3.14 uv run pytest tests/unit/test_etl_irs_soi_overlay.py tests/unit/test_etl_national_targets.py -q
• UV_PYTHON=3.14 uv run ruff check policyengine_us_data/db/etl_irs_soi.py policyengine_us_data/db/etl_national_targets.py tests/unit/test_etl_national_targets.py
• UV_PYTHON=3.14 uv run ruff format --check policyengine_us_data/db/etl_irs_soi.py policyengine_us_data/db/etl_national_targets.py tests/unit/test_etl_national_targets.py

[baogorek]

Mostly a "safe, mechanical refactor," but I'm requesting one line be removed (with .unique). Let me know if you want to discuss.

[baogorek]

Code smell for me. I really don't like deduplication commands unless there's a very good reason for them. From Claude:

  The thing is — this query has no .join(). It's a simple select(Stratum).where(...) with an .in_() filter.   
  There shouldn't be duplicate rows from that. The .unique() is unnecessary here.