refactor(migration): build pre-flight SQL via SQLAlchemy core (review)

Address Beto's review comments on apache/superset#39859: replace
``sa.text(f"...")`` SQL construction in the three pre-flight helpers
(``_delete_null_fk_rows``, ``_dedupe_by_min_id``, ``_assert_no_duplicates``)
with SQLAlchemy core constructs (``sa.delete``, ``sa.select``,
``sa.func``, ``.subquery()``, ``.notin_()``).

A small ``_table_clause()`` helper builds a lightweight ``TableClause``
exposing the columns the queries reference; the three helpers consume
it. Removes all ``# noqa: S608`` comments — they are no longer needed
because there is no string-interpolated SQL.

Verified the compiled SQL is identical on Postgres, MySQL, and SQLite,
including the MySQL ERROR 1093 workaround (the inner aggregation is
wrapped in a derived table via ``.subquery()``, producing
``... NOT IN (SELECT keep_id FROM (SELECT min(id) ...) AS keep_min)``).

Also drops the redundant ``f`` prefix on the two non-interpolating
lines of the ``_check_no_external_fks_to_id`` error message.

44 migration tests still pass.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>