chore: Add unit test for specific case and improve docstring

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

Author: agrawalradhika-cell

google/auth/transport/_mtls_helper.py

@@ -411,7 +411,8 @@ def check_use_client_cert():
     """Returns boolean for whether the client certificate should be used for mTLS.
 
     If GOOGLE_API_USE_CLIENT_CERTIFICATE is set to true or false, a corresponding
-    bool value will be returned.
+    bool value will be returned. If the value is set to an unexpected string, it
+    will default to False.
     If GOOGLE_API_USE_CLIENT_CERTIFICATE is unset, the value will be inferred
     by reading a file pointed at by GOOGLE_API_CERTIFICATE_CONFIG, and verifying
     it contains a "workload" section. If so, the function will return True,

tests/transport/test__mtls_helper.py

@@ -670,6 +670,11 @@ def test_check_use_client_cert_false(self, monkeypatch):
         use_client_cert = _mtls_helper.check_use_client_cert()
         assert use_client_cert is False
 
+    def test_check_use_client_cert_unsupported_value(self, monkeypatch):
+        monkeypatch.setenv("GOOGLE_API_USE_CLIENT_CERTIFICATE", "dummy")
+        use_client_cert = _mtls_helper.check_use_client_cert()
+        assert use_client_cert is False
+
     def test_check_use_client_cert_for_workload_with_config_file_not_found(
         self, monkeypatch
     ):