SecretManagerClient and ParameterManagerClient auth_token paths fail at runtime #5370

🔴 Required Information

Describe the Bug:
SecretManagerClient(auth_token=...) and ParameterManagerClient(auth_token=...) both fail immediately at runtime because they instantiate google.auth.credentials.Credentials(...) directly. That class is abstract, so the public auth_token path raises a TypeError before the underlying Google Cloud client is even constructed.

There is also a closely related contract mismatch in SecretManagerClient: its docstring says service_account_json and auth_token together should raise ValueError, but the implementation currently accepts both inputs and silently prefers service_account_json.

Steps to Reproduce:

1. Check out the current main branch (or latest stable v1.31.0).
2. Install the project dependencies (I reproduced this in a local checkout with Python 3.11.15).
3. Run the following snippet:

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()):
    SecretManagerClient(auth_token="test-token")

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

4. (Optional) Run this second snippet to see the conflicting-input behavior in SecretManagerClient:

import json
from unittest.mock import MagicMock, patch

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

with patch("google.cloud.secretmanager.SecretManagerServiceClient", return_value=MagicMock()), \
     patch("google.oauth2.service_account.Credentials.from_service_account_info", return_value=MagicMock()):
    SecretManagerClient(
        service_account_json=json.dumps({"type": "service_account"}),
        auth_token="test-token",
    )

Expected Behavior:

• auth_token should create a concrete credential object and allow both helpers to initialize successfully.
• SecretManagerClient should reject service_account_json + auth_token together, matching its own documented contract.

Observed Behavior:
The auth_token path fails with:

TypeError: Can't instantiate abstract class Credentials with abstract method refresh

And SecretManagerClient(service_account_json=..., auth_token=...) succeeds instead of raising.

Environment Details:

• ADK Library Version (pip show google-adk): reproduced on latest stable v1.31.0 and latest main (36ab8f128c0281e44c2120a17de91e081f2232b1 as of 2026-04-17)
• Desktop OS:** macOS
• Python Version (python -V): 3.11.15

Model Information:

• Are you using LiteLLM: No
• Which model is being used: N/A (this bug reproduces before any model call)

🟡 Optional Information

Regression:
These helpers were introduced recently and the broken behavior is still present in the latest stable release and latest main.

Logs:

TypeError: Can't instantiate abstract class Credentials with abstract method refresh

Additional Context:
The existing token-path unit tests currently mock the google.auth.credentials.Credentials constructor, which masks the runtime failure:

• tests/unittests/integrations/secret_manager/test_secret_client.py
• tests/unittests/integrations/parameter_manager/test_parameter_client.py

I have a local patch ready that switches both helpers to a concrete token credential type and adds regression coverage that uses the real credentials constructor path.

Minimal Reproduction Code:

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

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

How often has this issue occurred?:

• Always (100%)