fix(gateway): update Google model names in allowed custom chat models (#63109)

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

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## Test plan

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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",
  },
```

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cmd/frontend/internal/httpapi/completions/chat.go

@@ -86,9 +86,9 @@ func isAllowedCustomChatModel(model string, isProUser bool) bool {
 			"openai/gpt-4o",
 			"openai/gpt-4-turbo",
 			"openai/gpt-4-turbo-preview",
-			"google" + google.Gemini15FlashLatest,
-			"google" + google.Gemini15ProLatest,
-			"google" + google.GeminiProLatest,
+			"google/" + google.Gemini15FlashLatest,
+			"google/" + google.Gemini15ProLatest,
+			"google/" + google.GeminiProLatest,
 
 			// Remove after the Claude 3 rollout is complete
 			"anthropic/claude-2",