chore: complete SQLAlchemy 2.0 migration in coredns_mysql backend and tests ⬆️

Migrate remaining session.query() calls in coredns_mysql.py to
select()/session.execute() style; update bulk delete to delete()
construct and count to func.count(); drop sessionmaker(bind=).
Update test fixtures and assertions to match.

Zero session.query() calls remaining across the entire codebase.

tests/test_coredns_mysql.py

@@ -1,7 +1,7 @@
 """Tests for the CoreDNS MySQL backend (run against in-memory SQLite)."""
 
 import pytest
-from sqlalchemy import create_engine
+from sqlalchemy import create_engine, select
 from sqlalchemy.orm import scoped_session, sessionmaker
 
 from directdnsonly.app.backends.coredns_mysql import (
@@ -28,7 +28,7 @@ def mysql_backend():
             self.config = {}
             self.instance_name = "test"
             self.engine = engine
-            self.Session = scoped_session(sessionmaker(bind=engine))
+            self.Session = scoped_session(sessionmaker(engine))
 
     yield _TestBackend()
     engine.dispose()
@@ -84,8 +84,8 @@ def test_write_zone_removes_stale_records(mysql_backend):
     mysql_backend.write_zone("example.com", reduced)
 
     session = mysql_backend.Session()
-    zone = session.query(Zone).filter_by(zone_name="example.com.").first()
-    records = session.query(Record).filter_by(zone_id=zone.id, type="AAAA").all()
+    zone = session.execute(select(Zone).filter_by(zone_name="example.com.")).scalar_one_or_none()
+    records = session.execute(select(Record).filter_by(zone_id=zone.id, type="AAAA")).scalars().all()
     assert records == []
     session.close()
 
@@ -141,7 +141,7 @@ def test_reconcile_removes_extra_records(mysql_backend):
 
     # Inject a phantom record directly into the DB
     session = mysql_backend.Session()
-    zone = session.query(Zone).filter_by(zone_name="example.com.").first()
+    zone = session.execute(select(Zone).filter_by(zone_name="example.com.")).scalar_one_or_none()
     session.add(
         Record(
             zone_id=zone.id,