Make SQA store SQLAlchemy 2.0-compatible

[LeoMoonStar]

Summary:
The OSS Ax SQA store has a hard guard in with_db_settings_base.py that raises IncompatibleDependencyVersion when SQLAlchemy major > 1, disabling SQL storage entirely. This blocks SA 2.0 adoption (T163607006) and Python 3.13/3.14 (which auto-select SA 2.0.48 from third-party). Two additional SA 2.0 incompatibilities exist in OSS: defer("col_name") in load.py and reduced_state.py, which SA 2.0 rejects in favor of class-bound attribute references.

This diff removes the guard and converts the string-based loader options to attribute references. Adds a dual-version Buck test target tests_sa2 via constraint_overrides plus a self-proving TestSQLAlchemyDualVersionCompat class so each target proves its constraint took effect (EXPECTED_SA_MAJOR env var asserted at runtime).

Reviewed By: yangjoanna, andycylmeta

Differential Revision: D104875017

[meta-codesync]

@LeoMoonStar has exported this pull request. If you are a Meta employee, you can view the originating Diff in D104875017.

[codecov-commenter]

Codecov Report

❌ Patch coverage is 77.77778% with 4 lines in your changes missing coverage. Please review.
✅ Project coverage is 96.38%. Comparing base (14d3475) to head (a8c0860).

Files with missing lines	Patch %	Lines
...rage/sqa_store/tests/test_with_db_settings_base.py	80.00%	3 Missing ⚠️
ax/storage/sqa_store/with_db_settings_base.py	66.66%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #5201      +/-   ##
==========================================
- Coverage   96.38%   96.38%   -0.01%     
==========================================
  Files         617      617              
  Lines       69605    69615      +10     
==========================================
+ Hits        67090    67099       +9     
- Misses       2515     2516       +1     

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[meta-codesync]

This pull request has been merged in bd2ff97.