chore: Add mtls flag check

Signed-off-by: Radhika Agrawal <agrawalradhika@google.com>

Author: agrawalradhika-cell

tests/transport/aio/test_sessions_mtls.py

@@ -79,6 +79,7 @@ async def test_configure_mtls_channel_disabled(self, mock_exists):
         # If the file doesn't exist, it shouldn't error; it just won't use mTLS
         assert session._is_mtls is False
 
+    @mock.patch.dict(os.environ, {"GOOGLE_API_USE_CLIENT_CERTIFICATE": "true"})
     @mock.patch("os.path.exists")
     @mock.patch(
         "builtins.open", new_callable=mock.mock_open, read_data='{"invalid": "format"}'