Add service_account_json support to GoogleAdManagerAdapter validation

[bokelley]

Summary

Fixes "Failed to fetch advertisers" error by adding service_account_json support to GoogleAdManagerAdapter's config validation.

Problem

After fixing the credential wrapping issues (PRs #579, #581, #584), service account authentication was still failing when fetching advertisers with:

Failed to fetch advertisers: GAM config requires either 'service_account_key_file' or 'refresh_token'

Root Cause

GoogleAdManagerAdapter.__init__() was only checking for two authentication methods:

• service_account_key_file (legacy file-based service account)
• refresh_token (OAuth)

It was not recognizing service_account_json (JSON string-based service account, preferred for cloud deployments), even though:

• ✅ build_gam_config_from_adapter() correctly adds it to config
• ✅ GAMAuthManager correctly handles it
• ✅ GAMClientManager correctly uses it

The adapter was rejecting valid service account configs before they could reach the auth layer.

Solution

Updated GoogleAdManagerAdapter to recognize service_account_json:

Line 90: Store service_account_json from config

self.service_account_json = config.get("service_account_json")

Line 110-111: Update validation to accept all three methods

if not self.key_file and not self.service_account_json and not self.refresh_token:
    raise ValueError("GAM config requires either 'service_account_key_file', 'service_account_json', or 'refresh_token'")

Complete Service Account Fix

This completes the service account authentication implementation:

• ✅ PR Fix GAM service account authentication #579: Credential wrapping in auth manager
• ✅ PR Fix remaining service account credential wrapping issues #581: Credential wrapping in inventory sync & health checks
• ✅ PR Fix service account credential wrapping in GAM setup flow #584: Credential wrapping in GAM setup flow
• ✅ This PR: Config validation in GoogleAdManagerAdapter

All service account authentication paths now work end-to-end!

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

🤖 Generated with Claude Code

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