Google: Guard bigquery_to_postgres's psycopg2-specific adapter registration

[Dev-iL]

related:

• Remove psycopg2 and migrate the sync Postgres driver to psycopg3 #68453

depends on:

• Postgres: Make psycopg (v3) the default synchronous Postgres driver #69474

register_adapter(list/dict, Json) in BigQueryToPostgresOperator is psycopg2-specific and previously ran unconditionally, forcing psycopg2 to be importable even when the resolved PostgresHook connection is actually on psycopg3 (where it's a no-op). Only import and call it when PostgresHook is actually on the psycopg2 path.

Note: this module only imports cleanly without psycopg2 installed once apache-airflow-providers-postgres itself stops hard-requiring psycopg2 — that's the companion change in psycopg3-sync-postgres. This PR should be merged after (or at least released alongside) that one; see the code comment at the import site in bigquery_to_postgres.py.

What changed

• providers/google/src/airflow/providers/google/cloud/transfers/bigquery_to_postgres.py: the from psycopg2.extensions import register_adapter / from psycopg2.extras import Json imports and the register_adapter(...) calls are now conditional on not PostgresHook.USE_PSYCOPG3, guarded inside postgres_hook, with a comment documenting the dependency on the postgres provider's own guard fix.

Test plan

• Existing bigquery_to_postgres test suite passes.
• New tests: test_adapters_to_json_registered_on_psycopg2_path, test_adapters_not_registered_on_psycopg3_path, test_bigquery_to_postgres_module_imports_without_psycopg2 (simulates psycopg2 absent via sys.modules patching, confirms clean import).

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