OAuth 2.0 credentials ignored - _create_credentials() only uses google.auth.default()

[Teake1404]

Bug Description

The _create_credentials() function in ads_mcp/utils.py only uses google.auth.default() which looks for GCP Application Default Credentials (service accounts, gcloud auth). This ignores OAuth 2.0 credentials (client_id, client_secret, refresh_token) that are the standard authentication method for Google Ads API.

Steps to Reproduce

1. Configure MCP with valid OAuth 2.0 credentials in Claude Desktop config:

{
  "mcpServers": {
    "google-ads": {
      "command": "uvx",
      "args": ["google-ads-mcp"],
      "env": {
        "GOOGLE_ADS_DEVELOPER_TOKEN": "xxx",
        "GOOGLE_ADS_CLIENT_ID": "xxx.apps.googleusercontent.com",
        "GOOGLE_ADS_CLIENT_SECRET": "xxx",
        "GOOGLE_ADS_REFRESH_TOKEN": "xxx",
        "GOOGLE_ADS_LOGIN_CUSTOMER_ID": "xxx"
      }
    }
  }
}

Attempt to use any MCP tool
Get authentication error because google.auth.default() can't find GCP ADC
Expected Behavior
The MCP should use OAuth 2.0 credentials when provided via environment variables.

Suggested Fix
Replace google.auth.default() with GoogleAdsClient.load_from_dict():

def _create_credentials() -> GoogleAdsClient:
    credentials = {
        "developer_token": os.environ.get("GOOGLE_ADS_DEVELOPER_TOKEN"),
        "client_id": os.environ.get("GOOGLE_ADS_CLIENT_ID"),
        "client_secret": os.environ.get("GOOGLE_ADS_CLIENT_SECRET"),
        "refresh_token": os.environ.get("GOOGLE_ADS_REFRESH_TOKEN"),
        "login_customer_id": os.environ.get("GOOGLE_ADS_LOGIN_CUSTOMER_ID"),
        "use_proto_plus": True,
    }
    return GoogleAdsClient.load_from_dict(credentials)

Environment
google-ads-mcp version: latest (installed via uvx)
Python version: 3.14
OS: macOS