Replace Base.metadata.remove() hack with dedicated Edge3 declarative base (#62202)

* Replace Base.metadata.remove() hack with dedicated Edge3 declarative base

  Introduce models/edge_base.py with its own MetaData, registry, and
  Base for the Edge3 provider. All three Edge models now inherit from this
  isolated base, so their tables are never registered in Airflow core's
  Base.metadata in the first place — eliminating the need for the
  to_metadata() copy + Base.metadata.remove() workaround in db.py.

* Fix tests

* Add unit test for edge base