integrations: gcs, secret_manager and parameter_manager import google-cloud clients at module level, violating the lazy-import contract #7580

Describe the bug

integrations/README.md requires each integration to import its optional third-party dependency lazily and raise a descriptive error pointing at the extra to install. Three GCP integrations do not follow this:

• google/adk/integrations/gcs/client.py - from google.cloud import storage and import google.api_core.client_info at module level
• google/adk/integrations/secret_manager/secret_client.py - from google.cloud import secretmanager and from google.api_core.gapic_v1 import client_info at module level
• google/adk/integrations/parameter_manager/parameter_client.py - from google.cloud import parametermanager_v1 and from google.api_core.gapic_v1 import client_info at module level

google-cloud-storage, google-cloud-secret-manager, google-cloud-parametermanager and google-api-core are only in the gcp / all extras, not core dependencies.

To Reproduce

python -m venv v && v/bin/pip install google-adk
v/bin/python -c "import google.adk.integrations.secret_manager"
# ModuleNotFoundError: No module named 'google.api_core'

Same for google.adk.integrations.parameter_manager and google.adk.integrations.gcs.

Expected behavior

The modules import without the gcp extra, and the first use (constructing SecretManagerClient / ParameterManagerClient, calling get_gcs_client) raises an ImportError that says to run pip install 'google-adk[gcp]', like integrations/model_armor and integrations/mongodb already do.

Desktop

• google-adk 2.12.0, Python 3.11

I am working on a fix (Googler, change coming via the internal repo).