Support SQLAlchemy 2.0 in FB storage extensions

[LeoMoonStar]

Summary:
The Meta-internal Ax storage extensions in ax/fb/storage/ have two SA 2.0 incompatibilities not present in the OSS surface: a raw SQL string passed to session.execute in fb sqa_store db.py (SA 2.0 requires text() wrapping), and external_store.py uses Connection.execute() for writes without explicit transaction (SA 2.0 removed implicit autocommit, so writes were silently rolling back), uses string-keyed Row indexing (SA 2.0 requires row._mapping[key]), and consumes a Result generator outside the connection context (SA 2.0 closes the Result on connection close).

This diff wraps SHOW DATABASES with text(), switches _write to engine.begin() for transactional commit, migrates _decode_row to row._mapping access, and materializes the read_raw_data result list inside the with conn block. Adds tests_sa2 dual-version Buck targets for fb sqa_store, fb external_store, and fb prod_tests, plus a SQLAlchemy2CompatTest smoke test that exercises the libfb.py.db_locator -> creator -> engine -> session -> SELECT 1 path and asserts EXPECTED_SA_MAJOR.

Differential Revision: D104875016

[meta-codesync]

@LeoMoonStar has exported this pull request. If you are a Meta employee, you can view the originating Diff in D104875016.

[codecov-commenter]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 96.38%. Comparing base (bd2ff97) to head (bbea32b).

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #5203   +/-   ##
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  Coverage   96.38%   96.38%           
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  Files         617      617           
  Lines       69615    69615           
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  Hits        67100    67100           
  Misses       2515     2515           

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