Fix async database engine setup for cockroachdb connection URIs

[viragtripathi]

SQLAlchemy's registered cockroachdb dialect speaks the PostgreSQL wire protocol and uses asyncpg as its async driver. Airflow 3.x derives the async connection URI from the sync one via AIO_LIBS_MAPPING in settings.py; because cockroachdb is not in that map, startup fails with "The loaded 'psycopg2' is not async" unless the user hand-writes
sql_alchemy_conn_async. This adds the one mapping entry and a parametrized test for the derivation function, which previously had no direct coverage.

Discussed on the devlist:
https://lists.apache.org/thread/t6jo4th3sn23jmr34m6gcxzw4k8mo4pc

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Was generative AI tooling used to co-author this PR?

• Yes - Claude Code

Generated-by: Claude Code following the guidelines

[boring-cyborg]

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[ashb]

Putting a temporary hold while we decide what we want to do -- see https://lists.apache.org/thread/qt7ohpwop8jgos1o8fszl4rv1ddkpz6v