test: cover GizmoSQL native metadata fallback

[cofin]

Summary

• add live GizmoSQL/FlightSQL GetObjects coverage for catalog/schema/table/column payload shape
• document that DuckDB-backed GizmoSQL native metadata is used only when complete, with central dialect SQL fallback for incomplete constraints/nullability
• keep the follow-up separate from merged PR feat: add native metadata statistics #522

Validation

• uv run pytest tests/unit/adapters/test_adbc/test_data_dictionary.py tests/integration/adapters/adbc/test_gizmosql_data_dictionary.py (26 passed, 1 xfailed)
• make docs
• make lint

[codecov-commenter]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 75.24%. Comparing base (fc54103) to head (d45c4c0).
⚠️ Report is 1 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #534      +/-   ##
==========================================
- Coverage   75.24%   75.24%   -0.01%     
==========================================
  Files         444      444              
  Lines       58121    58121              
  Branches     8998     8998              
==========================================
- Hits        43732    43731       -1     
- Misses      11562    11563       +1     
  Partials     2827     2827              

Flag	Coverage Δ
integration	59.48% <ø> (-0.03%)	⬇️
py3.10	73.59% <ø> (-0.01%)	⬇️
py3.11	73.60% <ø> (-0.01%)	⬇️
py3.12	73.60% <ø> (-0.01%)	⬇️
py3.13	73.60% <ø> (-0.01%)	⬇️
py3.14	74.41% <ø> (-0.01%)	⬇️
unit	60.74% <ø> (+<0.01%)	⬆️

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see 2 files with indirect coverage changes

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