fix: Add test cases for helper method

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

Author: agrawalradhika-cell

tests/transport/test_aio_mtls_helper.py

@@ -0,0 +1,88 @@
+import os
+import pytest
+from unittest import mock
+from google.auth import exceptions
+# Assuming the provided code is in a file named google/auth/transport/aio/mtls_helper.py
+from google.auth.transport.aio import mtls_helper
+
+CERT_DATA = b"client-cert"
+KEY_DATA = b"client-key"
+
+class TestMTLSHelper:
+
+    @mock.patch("os.path.expanduser")
+    @mock.patch("os.path.exists")
+    def test__check_config_path_exists(self, mock_exists, mock_expand):
+        mock_expand.side_effect = lambda x: x.replace("~", "/home/user")
+        mock_exists.return_value = True
+        
+        path = "/home/user/config.json"
+        result = mtls_helper._check_config_path("~/config.json")
+        
+        assert result == path
+        mock_exists.assert_called_with(path)
+
+    @mock.patch("os.path.exists", return_value=False)
+    def test__check_config_path_not_found(self, mock_exists):
+        result = mtls_helper._check_config_path("nonexistent.json")
+        assert result is None
+
+    @mock.patch("google.auth.transport.aio.mtls_helper._check_config_path")
+    @mock.patch("os.getenv")
+    def test_has_default_client_cert_source_default_path(self, mock_getenv, mock_check):
+        # Case 1: Default config path exists
+        mock_check.side_effect = lambda x: x if x == mtls_helper.CERTIFICATE_CONFIGURATION_DEFAULT_PATH else None
+        
+        assert mtls_helper.has_default_client_cert_source() is True
+
+    @mock.patch("google.auth.transport.aio.mtls_helper._check_config_path")
+    @mock.patch("os.getenv")
+    def test_has_default_client_cert_source_env_var(self, mock_getenv, mock_check):
+        # Case 2: Default path doesn't exist, but env var path does
+        custom_path = "/custom/path.json"
+        mock_check.side_effect = lambda x: x if x == custom_path else None
+        mock_getenv.return_value = custom_path
+        
+        assert mtls_helper.has_default_client_cert_source() is True
+
+    @mock.patch("google.auth.transport.aio.mtls_helper._check_config_path", return_value=None)
+    def test_has_default_client_cert_source_none(self, mock_check):
+        assert mtls_helper.has_default_client_cert_source() is False
+
+    @mock.patch("google.auth.transport._mtls_helper._get_workload_cert_and_key")
+    def test_get_client_ssl_credentials_success(self, mock_workload):
+        mock_workload.return_value = (CERT_DATA, KEY_DATA)
+        
+        success, cert, key, passphrase = mtls_helper.get_client_ssl_credentials()
+        
+        assert success is True
+        assert cert == CERT_DATA
+        assert key == KEY_DATA
+        assert passphrase is None
+
+    @mock.patch("google.auth.transport._mtls_helper._get_workload_cert_and_key", return_value=(None, None))
+    def test_get_client_ssl_credentials_fail(self, mock_workload):
+        success, cert, key, passphrase = mtls_helper.get_client_ssl_credentials()
+        assert success is False
+        assert cert is None
+
+    def test_get_client_cert_and_key_callback(self):
+        # Callback should take priority
+        callback = mock.Mock(return_value=(CERT_DATA, KEY_DATA))
+        
+        success, cert, key = mtls_helper.get_client_cert_and_key(callback)
+        
+        assert success is True
+        assert cert == CERT_DATA
+        assert key == KEY_DATA
+        callback.assert_called_once()
+
+    @mock.patch("google.auth.transport.aio.mtls_helper.get_client_ssl_credentials")
+    def test_get_client_cert_and_key_default(self, mock_get_ssl):
+        mock_get_ssl.return_value = (True, CERT_DATA, KEY_DATA, None)
+        
+        success, cert, key = mtls_helper.get_client_cert_and_key(None)
+        
+        assert success is True
+        assert cert == CERT_DATA
+        assert key == KEY_DATA