Cody Gateway: add gemini model to code completion list (#63278)

Add a stable version of the Gemini 1.5 Flash model to the allowed list
for Code Completion.

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## Test plan

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1. Start a local Cody Gateway instance
2. In your local sg instance set "google/gemini-1.5-flash" as your
completionModel
3. curl to the completion endpoint to confirm you are getting a
completion response instead of streaming response (starts with data:):
```
{"completion":"I am a large language model, trained by Google. \n\nHere's what that means:\n\n* **Large Language Model:** I'm","stopReason":"STOP"}%
```

## Changelog

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cmd/frontend/internal/dotcom/productsubscription/codygateway_dotcom_user.go

@@ -333,6 +333,7 @@ var allCodeCompletionModels = slices.Concat([]string{"anthropic/" + anthropic.Cl
 	"anthropic/claude-instant-1",
 	"anthropic/claude-instant-1.2-cyan",
 	"anthropic/claude-instant-1.2",
+	"google/" + google.Gemini15Flash,
 	"fireworks/starcoder",
 	"fireworks/" + fireworks.Llama213bCode,
 	"fireworks/" + fireworks.StarcoderTwo15b,