fix: support pool_pre_ping in SQLAlchemy ORM MySQL dialect

[sudosubin]

Description

engine = create_engine(
    "mysql+aws_wrapper_mysqlconnector://user:pass@host:3306/db",
    pool_pre_ping=True,
)

Fixes pool_pre_ping=True for the SQLAlchemy ORM MySQL dialect. SQLAlchemy passes the DBAPI connection to do_ping, which here is an AwsWrapperConnection. The inherited do_ping calls dbapi_connection.ping(), but AwsWrapperConnection does not expose ping(), so every pool checkout raised 'AwsWrapperConnection' object has no attribute 'ping'.

This overrides do_ping to ping the wrapped driver connection (target_connection) instead. A dead connection raises a native mysql.connector error, which SQLAlchemy cannot classify as a disconnect because import_dbapi reports the wrapper module as the DBAPI (its exception hierarchy is unrelated to mysql.connector's), so do_ping catches it and returns False, letting the pool invalidate and reconnect.

Unit tests are added for the live and dead connection paths.

By submitting this pull request, I confirm that my contribution is made under the terms of the Apache 2.0 license.