Fix: Correct AdManagerClient signature for service account auth

[kmoegling-scope3]

Problem

PR #570 was merged but inventory sync still fails for service account authentication with error:

Sync failed: GAM error: 'Credentials' object has no attribute 'CreateHttpHeader'

Affected tenant: weather at https://sales-agent.scope3.com/admin/tenant/weather/inventory?type=all

Root Cause

The AdManagerClient constructor has different signatures depending on credential type:

1. OAuth credentials (googleads.oauth2.GoogleRefreshTokenClient):

   • Has CreateHttpHeader method
   • Signature: AdManagerClient(oauth_client, "App Name", network_code=...)

2. Service account credentials (google.oauth2.service_account.Credentials):

   • Does NOT have CreateHttpHeader method (it's from google-auth, not googleads)
   • Signature: AdManagerClient(credentials, network_code) (2 args only)

Solution

Changed service account AdManagerClient initialization from 3 args to 2 args (line 576):

- client = ad_manager.AdManagerClient(
-     oauth2_credentials, "AdCP Sales Agent", network_code=adapter_config.gam_network_code
- )
+ client = ad_manager.AdManagerClient(oauth2_credentials, adapter_config.gam_network_code)

OAuth path remains unchanged (line 596) - correctly uses 3 arguments.

Evidence

GAM health check (src/adapters/gam/utils/health_check.py:86) uses the same 2-argument pattern:

self.client = ad_manager.AdManagerClient(oauth2_credentials, self.config.get("network_code"))

Testing

• ✅ Service account auth: Now uses correct 2-arg signature
• ✅ OAuth auth: Unchanged, continues to use 3-arg signature
• ✅ Matches established pattern from GAM health check

Impact

• Fixes inventory sync for service account tenants (e.g., weather)
• No impact on OAuth tenants (code unchanged)
• Low risk: One-line change, matches working pattern elsewhere

Fixes #570