Detect augmented JSON types when skipping batch migrate CAST

[ArockiaRajamanickam]

Fixes the data loss reported in #1120.

cast_for_batch_migrate skips the data transfer CAST for JSON, because CASTing to JSON in SQLite yields 0. The check was isinstance(new_type, JSON), which a TypeDecorator wrapping JSON does not satisfy, so the CAST went out and every value in the column became 0.

Reproduced on an in-memory SQLite database with one row holding {"keep": "me", "n": 123}:

control, sa.JSON      value after: '{"keep": "me", "n": 123}'
                      INSERT INTO _alembic_tmp_bar (foo) SELECT bar.foo FROM bar

TypeDecorator(JSON)   value after: 0
                      INSERT INTO _alembic_tmp_bar (foo) SELECT CAST(bar.foo AS JSON) AS foo FROM bar

The change compares type affinity, which the condition on the line above already does:

existing.type._type_affinity is not new_type._type_affinity
and new_type._type_affinity is not JSON

One note on the approach suggested in the issue. isinstance(typ.dialect_impl(dialect), JSON) does not catch this case, because for a TypeDecorator dialect_impl() returns the decorator itself rather than its impl. I measured each candidate against the SQLite dialect:

type	isinstance	dialect_impl	_type_affinity is JSON
sa.JSON()	True	True	True
TypeDecorator(impl=JSON)	False	False	True
sqlite.JSON()	True	True	True
postgresql.JSONB()	True	True	True
String, Integer, TypeDecorator(impl=String)	False	False	False

Affinity is the only one that catches the TypeDecorator while still excluding non-JSON types. impl_instance also works, but only for the decorator, so it would have to be combined with the existing isinstance check.

Worth flagging: this is a behaviour change in one narrow direction. CAST suppression now covers any type whose affinity is JSON, not only JSON instances, so a change from a genuinely different affinity to an augmented JSON type no longer emits the CAST. That is what the original guard was for, just applied to the augmented case too.

The test is tests/test_batch.py::CopyFromTest::test_change_type_json_typedecorator, next to test_change_type as suggested in the issue. It fails on main with CAST(foo.toj AS JSON) AS toj in the transfer statement and passes with the fix. Full suite is 1770 passed, 133 skipped, and there is a changelog fragment in docs/build/unreleased/1120.rst.

I used an AI assistant while working on this. The reproduction and the comparison table are things I ran myself.