fix(auth): use an unencrypted key for mTLS certificate rotation

[Thanatos9404]

call_client_cert_callback() in google/auth/transport/_mtls_helper.py called get_client_ssl_credentials(generate_encrypted_key=True) and then dropped the passphrase. With SecureConnect (context_aware_metadata.json) that adds --with_passphrase to the cert provider command, so the key it returns is encrypted.

Its only caller is check_parameters_for_unauthorized_response(), which the 401 certificate rotation in AuthorizedSession.request() and AuthorizedHttp.urlopen() uses. Both pass the key to configure_mtls_channel() without a passphrase, so load_cert_chain() gets an encrypted key with password=None.

This passes generate_encrypted_key=False, as get_client_cert_and_key() already does for the initial mTLS setup, and updates test_call_client_cert_callback to expect it. A new test drives the SecureConnect path through a fake cert provider that returns an encrypted key only for --with_passphrase, and checks that the returned key loads without a passphrase. Workload certificates from certificate_config.json are returned before the flag is read, so they are not affected, and the async rotation path does not use this function.

The change does not touch requests.py or urllib3.py, so it is independent of #18381. The rotation paths added there also go through check_parameters_for_unauthorized_response() and get the unencrypted key too.

Testing (Python 3.13 on Windows, Python 3.12 on Linux):

• The updated test_call_client_cert_callback and the new test_call_client_cert_callback_secure_connect_unencrypted_key fail against _mtls_helper.py from main (the first with Expected: get_client_ssl_credentials(generate_encrypted_key=False), Actual: ...(generate_encrypted_key=True); the second gets the encrypted key back) and pass with the change.

• pytest tests/transport/test__mtls_helper.py tests/transport/test_requests.py tests/transport/test_urllib3.py tests/transport/test_mtls.py: 371 passed.

• pytest tests tests_async on Linux: 2263 passed, 7 skipped.

• ruff check --select I and ruff format --check (ruff 0.14.14 with the flags from the lint session) and flake8 on the two changed files: no findings.

• On Linux, passing an encrypted key with no passphrase to _MutualTlsAdapter with no terminal attached printed Enter PEM pass phrase: and raised MutualTLSChannelError (caused by OSError(22, 'Invalid argument')); with a terminal it waits for input. The same key unencrypted loads fine.

• Make sure to open an issue as a bug/issue before writing your code! That way we can discuss the change, evaluate designs, and agree on the general idea

• Ensure the tests and linter pass

• Code coverage does not decrease (if any source code was changed)

• Appropriate docs were updated (if necessary)

Fixes #18467 🦕.

[gemini-code-assist]

Code Review

This pull request modifies call_client_cert_callback in _mtls_helper.py to request an unencrypted key by setting generate_encrypted_key=False when calling get_client_ssl_credentials, as callers load the key without a passphrase. It also updates existing tests and adds a new test test_call_client_cert_callback_secure_connect_unencrypted_key to verify this behavior. No review comments were provided, and I have no additional feedback.