Migrate Airflow test-connection to the @check framework

[IceS2]

Migrate the Airflow connector's test-connection onto the declarative @check / ChecksProvider / ErrorPack framework, matching the connectors already migrated (dbt Cloud, Databricks, Tableau).

Airflow is the hybrid case: the owned client is an AirflowApiClient (REST) or a SQLAlchemy Engine (metadata-DB backend). The checks provider must borrow that client from BaseConnection (Borrowed[Any]) and dispatch on its type — it must never build or authenticate a second client.

Steps: CheckAccess (gate), PipelineDetailsAccess, TaskDetailAccess.