Fix GAM service account authentication

[bokelley]

Summary

Fixes 400 errors when calling GAM API with service account authentication by wrapping credentials in the required GoogleCredentialsClient wrapper.

Problem

AdManagerClient requires a googleads.oauth2.GoogleOAuth2Client subclass, but the code was passing raw google.oauth2.service_account.Credentials objects directly, causing 400 errors with incorrect argument signatures.

Solution

• Wrap google.oauth2.service_account.Credentials in oauth2.GoogleCredentialsClient before returning from _get_service_account_credentials()
• Apply fix to both JSON string and file-based service account credential paths
• Update tests to verify returned object is GoogleCredentialsClient
• Add test to confirm OAuth2 client compatibility with AdManagerClient

Changes

File: src/adapters/gam/auth.py

• Line 100: Wrap credentials in oauth2.GoogleCredentialsClient() for JSON string path
• Line 111: Wrap credentials in oauth2.GoogleCredentialsClient() for file path

File: tests/unit/test_gam_service_account_auth.py

• Update test_service_account_credentials_creation to verify GoogleCredentialsClient wrapper
• Add test_service_account_returns_compatible_oauth2_client to verify GoogleOAuth2Client inheritance

Test Results

✅ All 8 service account auth tests pass
✅ All 35 GAM unit tests pass
✅ All 846 unit tests pass
✅ All 174 integration tests pass

References

• Google Ad Manager API Authentication Docs
• googleads-python-lib Wiki: Service Account Flow

🤖 Generated with Claude Code

Co-Authored-By: Claude noreply@anthropic.com