Fix service account credential wrapping in GAM setup flow

[bokelley]

Summary

Fixes the final location where service account credentials weren't being wrapped, causing CreateHttpHeader errors during GAM tenant setup.

Problem

After PRs #579 and #581 fixed the auth manager, inventory sync, and health checks, service account authentication was still failing during GAM tenant setup (network detection and advertiser/user fetching) with:

'Credentials' object has no attribute 'CreateHttpHeader'

Root Cause

One more location in src/admin/blueprints/gam.py (line 1189-1192) was creating raw google.oauth2.service_account.Credentials and passing them directly to AdManagerClient without wrapping them in GoogleCredentialsClient.

This code is used during the tenant setup flow when:

• Detecting available GAM networks
• Fetching advertisers and users for initial configuration

Solution

Wrapped the credentials in oauth2.GoogleCredentialsClient at line 1189-1194:

# Before (WRONG):
oauth2_client = service_account.Credentials.from_service_account_info(...)

# After (CORRECT):
credentials = service_account.Credentials.from_service_account_info(...)
oauth2_client = oauth2.GoogleCredentialsClient(credentials)

Comprehensive Fix Complete

This completes the service account authentication fix across all locations in the codebase:

• ✅ PR Fix GAM service account authentication #579: src/adapters/gam/auth.py (auth manager)
• ✅ PR Fix remaining service account credential wrapping issues #581: src/admin/blueprints/inventory.py (inventory sync)
• ✅ PR Fix remaining service account credential wrapping issues #581: src/adapters/gam/utils/health_check.py (health checks)
• ✅ This PR: src/admin/blueprints/gam.py (tenant setup)

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

Verification

Searched entire codebase - all instances of service_account.Credentials.from_service_account_* are now properly wrapped in GoogleCredentialsClient before being passed to AdManagerClient.

🤖 Generated with Claude Code

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