Allow upgrades with NUL in serialized Dags

[dhkim1920]

PostgreSQL accepts U+0000 escapes in JSON, but rejects them when migration 0089 casts serialized_dag.data to JSONB. This blocks upgrades from Airflow 3.1 when a serialized Dag contains a NUL character.

This draft applies the sanitization approach established by #69064 to migration 0089:

• protect escaped backslashes so JSON strings containing the literal escape spelling are preserved;
• strip active U+0000 escapes before the JSONB cast;
• add a PostgreSQL regression test covering both cases.

Decision needed

#69064 strips U+0000 from historical XCom and DagRun data because PostgreSQL JSONB cannot represent it. Applying the same policy to serialized_dag is semantically different: a delimiter such as "@@\0@@" becomes "@@@@", changing the Dag definition.

This draft asks whether lossy sanitization is acceptable for this migration or whether serialized Dag data needs a different policy. Migration-only sanitization may also not address a later reserialization of the same Dag into JSONB, so the final scope depends on that policy decision.

closes: #65379

Tests

• breeze run --backend postgres pytest airflow-core/tests/unit/migrations/test_0089_change_serialized_dag_data_column_to_jsonb.py -xvs
• prek run --from-ref main --stage pre-commit
• prek run --from-ref main --stage manual --skip compile-ui-assets-dev --skip view-skill-eval
• breeze testing core-tests --run-in-parallel

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