fix: incorrect failover error rewrapping

[karenc-bq]

Description

3 fixes: one to how failover errors surface to SQLAlchemy consumers, and two that block upgrading from 3.0.0.

1. Failover errors reach SQLAlchemy unchanged

The dialect mixin caught FailoverSuccessError in do_execute / do_executemany and re-raised pep249.OperationalError. The substitution was unnecessary, and it broke two things.

It was unnecessary because SQLAlchemy classifies by walking orig.__class__.__mro__ and matching bases by class name against sqlalchemy.exc. FailoverError already derives from the wrapper's pep249.OperationalError, so the family already mapped to sqlalchemy.exc.OperationalError without any rewrap. Measured both ways: same sqlalchemy.exc class, differing only in .orig.

What it broke:

1. DBAPIError.orig became the substitute, so isinstance(err.orig, FailoverSuccessError) stopped matching. 3.0.0's shipped example used that check.
2. is_disconnect no longer received a FailoverError, so it delegated to MySQLDialect_mysqlconnector.is_disconnect, which reads e.errno. The wrapper's PEP-249 errors have no errno, and SQLAlchemy calls is_disconnect at the top of Connection._handle_dbapi_exception outside any try, so the resulting AttributeError escaped and the application's except DBAPIError never ran.

Reproduced on a real Aurora MySQL failover: a successful failover surfaced to the consumer as AttributeError: 'OperationalError' object has no attribute 'errno'. The connection was usable; only the signal telling the application to retry was lost. docs/examples/MySQLSQLAlchemyFailover.py catches OperationalError, which that bypasses.

The rewrap also made is_disconnect unreachable on the do_execute path, where it is needed. Removing it restores the override that keeps a recoverable failover from invalidating the pooled connection.

This change also defaults errno and sqlstate to None on pep249.Error. Because dialect.loaded_dbapi is this package, SQLAlchemy's MySQL dialect probes e.errno on anything that is an OperationalError or InterfaceError. Nine wrapper errors qualify: the FailoverError family plus QueryTimeoutError, AwsConnectError and ReadWriteSplittingError. Each was one unguarded probe away from the same AttributeError.

2. Type stubs are no longer runtime dependencies

boto3-stubs and types_aws_xray_sdk were declared in [tool.poetry.dependencies], which made upgrading impossible for some consumers.

3. The pre-3.1.0 dialect import path resolves again

SqlAlchemyOrmMysqlDialect moved to sqlalchemy_dialects/mysql.py::AwsWrapperMySQLConnectorDialect without an alias, so code that imported the class directly broke with ModuleNotFoundError. URL-based configuration was unaffected: the driver name aws_wrapper_mysqlconnector did not change. This adds a shim for the direct-import case, with a DeprecationWarning.

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