fix: restore auth_token initialization for secret and parameter manager clients

[shaun0927]

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Link to Issue or Description of Change

1. Link to an existing issue (if applicable):

• Closes: SecretManagerClient and ParameterManagerClient auth_token paths fail at runtime #5370

2. Or, if no issue exists, describe the change:

Problem:
Both SecretManagerClient and ParameterManagerClient advertise an auth_token constructor path, but the implementation instantiates google.auth.credentials.Credentials(...) directly. That base class is abstract, so callers hit a TypeError before the underlying Google Cloud client is constructed.

SecretManagerClient also documents that service_account_json and auth_token are mutually exclusive, but the current implementation silently accepts both inputs and ignores the token.

Solution:

• Switch both helpers to google.oauth2.credentials.Credentials(token=auth_token) for the auth_token path.
• Keep the change narrow to the validated runtime regression and its adjacent contract mismatch.
• Replace the mocked-constructor token-path tests with regression coverage that exercises the real credentials constructor path.
• Add a SecretManagerClient test that confirms conflicting credential inputs raise ValueError.

Testing Plan

Unit Tests:

• I have added or updated unit tests for my change.
• All unit tests pass locally.

Passed locally:

.venv/bin/pytest tests/unittests/integrations/secret_manager/test_secret_client.py tests/unittests/integrations/parameter_manager/test_parameter_client.py -q
17 passed, 1 warning in 24.43s

Manual End-to-End (E2E) Tests:

I also re-ran the original local reproductions with patched cloud clients to confirm the behavior change:

from unittest.mock import MagicMock, patch
from google.adk.integrations.secret_manager.secret_client import SecretManagerClient
from google.adk.integrations.parameter_manager.parameter_client import ParameterManagerClient

with patch("google.cloud.secretmanager.SecretManagerServiceClient", return_value=MagicMock()):
    client = SecretManagerClient(auth_token="test-token")
    assert client._credentials.token == "test-token"

with patch("google.cloud.parametermanager_v1.ParameterManagerClient", return_value=MagicMock()):
    client = ParameterManagerClient(auth_token="test-token")
    assert client._credentials.token == "test-token"

And for the conflicting-input case:

import json
from google.adk.integrations.secret_manager.secret_client import SecretManagerClient

try:
    SecretManagerClient(
        service_account_json=json.dumps({"type": "service_account"}),
        auth_token="test-token",
    )
except ValueError:
    pass
else:
    raise AssertionError("Expected conflicting credentials to raise ValueError")

Checklist

• I have read the CONTRIBUTING.md document.
• I have performed a self-review of my own code.
• I have commented my code, particularly in hard-to-understand areas.
• I have added tests that prove my fix is effective or that my feature works.
• New and existing unit tests pass locally with my changes.
• I have manually tested my changes end-to-end.
• Any dependent changes have been merged and published in downstream modules.

Additional context

I intentionally kept this PR focused on the reproducible auth helper regressions that are still present in the latest stable release and current main.

[rohityan]

Hi @shaun0927 , Thank you for your contribution! We appreciate you taking the time to submit this pull request. Please fix formatting errors by running autoformat.sh