Fix: Add missing / to provider name for google models in Cody Gateway allowlist.

I was getting the following error from Cody Gateway:

event: error
data: {"error":"Sourcegraph Cody Gateway: unexpected status code 400: {\"error\":\"model \\\"google/\\\" is not allowed, allowed: [google/gemini-1.5-pro-latest, google/gemini-1.5-flash-latest]\"}\n"}

Changes:

• prepend "google/" to the Google model names to match the expected format

Test plan

The issue doesn't affect instances that set google as the provider, and their requests should work as expected.

This change only affect Cody Gateway users with the following setting:

"completions": {
    "provider": "sourcegraph"
    "chatModel": "google/gemini-1.5-pro-latest",
    "fastChatModel": "google/gemini-1.5-flash-latest",
  },

Changelog