Set CosmosPlugin to None for airflow 3

cosmos/plugin/__init__.py

@@ -1,11 +1,19 @@
+from __future__ import annotations
+
+from typing import TYPE_CHECKING
+
 from airflow import __version__ as airflow_version
 from packaging import version
 
 from cosmos.constants import _AIRFLOW3_MAJOR_VERSION
 
+if TYPE_CHECKING:  # pragma: no cover
+    from .plugin_impl import CosmosPlugin as _CosmosPluginType
+
+CosmosPlugin: _CosmosPluginType | None = None
 # The plugin is only loaded if the Airflow version is less than 3.0. This is because the plugin is incompatible with
 # Airflow 3.0 and above. Once the compatibility issue is resolved as part of
 # https://github.com/astronomer/astronomer-cosmos/issues/1587, the import statement can be moved outside of the
 # conditional block.
 if version.parse(airflow_version).major < _AIRFLOW3_MAJOR_VERSION:
-    from .plugin_impl import CosmosPlugin as CosmosPlugin
+    from .plugin_impl import CosmosPlugin as CosmosPlugin  # type: ignore[assignment]

tests/plugin/test_plugin.py

@@ -380,3 +380,7 @@ def test_has_access_with_permissions_in_astro_must_include_custom_menu(url_path,
     mock_check_auth = cosmos.plugin.plugin_impl.dbt_docs_view.appbuilder.sm.check_authorization
     app.get(url_path)
     assert mock_check_auth.call_args[0][0] == [("menu_access", "Custom Menu"), ("can_read", "Website")]
+
+
+def test_cosmos_plugin_enabled_on_airflow2():
+    assert cosmos.plugin.CosmosPlugin is not None

tests/plugin/test_plugin_af3.py

@@ -0,0 +1,16 @@
+# This module is added specifically to test that the plugin is disabled in Airflow 3.x.
+# Currently, we skip the entire test_plugin.py module since it contains multiple tests that are only compatible with Airflow 2.x.
+# Adding this test to that file would require selectively applying pytest.mark.skipif to each individual test,
+# which would add unnecessary complexity.
+# To keep things straightforward, we isolate the one relevant test for Airflow 3 here,
+# while continuing to skip the existing plugin test module entirely for Airflow 3.x.
+import pytest
+from airflow import __version__ as airflow_version
+from packaging import version
+
+import cosmos.plugin
+
+
+@pytest.mark.skipif(version.parse(airflow_version).major == 2, reason="Plugin is available in Airflow 2.x")
+def test_cosmos_plugin_disabled_on_airflow3():
+    assert cosmos.plugin.CosmosPlugin is None