Document that connection secrets without conn_type remain supported

shared/secrets_backend/src/airflow_shared/secrets_backend/base.py

@@ -122,6 +122,13 @@ def _get_connection_class(self) -> type:
     def _deserialize_connection_value(conn_class: type, conn_id: str, value: str):
         value = value.strip()
         if value[0] == "{":
+            # JSON secrets stored by Airflow 2 backends may lack both "conn_type"
+            # and "uri" (e.g. {"host": ..., "login": ..., "password": ...}). This is
+            # valid: conn_type is intentionally optional on the SDK Connection model
+            # for Airflow 2 -> 3 migration compatibility.
+            # Check: https://github.com/apache/airflow/pull/61728
+            # TODO: Remove this compatibility once the minimum supported Airflow
+            #   version in providers is 3.0.
             return conn_class.from_json(value=value, conn_id=conn_id)  # type: ignore[attr-defined]
 
         # TODO: Only sdk has from_uri defined on it. Is it worthwhile developing the core path or not?

shared/secrets_backend/tests/secrets_backend/test_base.py

@@ -124,6 +124,21 @@ def test_deserialize_connection_json(self, sample_conn_json):
         assert conn.conn_id == "test_conn"
         assert conn._kwargs["conn_type"] == "mysql"
 
+    def test_deserialize_connection_json_without_conn_type(self):
+        """
+        Guards the Airflow 2 -> 3 migration compatibility established in
+        https://github.com/apache/airflow/pull/61728.
+        """
+        backend = _TestBackend()
+        backend._set_connection_class(MockConnection)
+
+        conn = backend.deserialize_connection(
+            "test_conn", '{"host": "example.com", "login": "admin", "password": "secret"}'
+        )
+        assert isinstance(conn, MockConnection)
+        assert conn.conn_id == "test_conn"
+        assert conn._kwargs == {"host": "example.com", "login": "admin", "password": "secret"}
+
     def test_deserialize_connection_uri(self, sample_conn_uri):
         """Test deserialize_connection with URI format through _TestBackend."""
         backend = _TestBackend()