feat(cody gateway): code completion support for Google Gemini Flash models (#63303)

- Add support for the latest versions of the Google Gemini models:
  - `google/Gemini15FlashLatest`
- `google/GeminiProLatest` (Gemini 1.5 Pro is not on the support list
for code completion)
- This allows users to access the most up-to-date versions of these
models for code completion when the client side support this later.

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## Test plan

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No features were changed.

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feat(cody gateway): code completion support for Google Gemini Flash
models

Author: abeatrix

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

@@ -93,7 +93,9 @@ func isAllowedCodeCompletionModel(model string) bool {
 		"anthropic/claude-instant-1",
 		"anthropic/claude-instant-1.2-cyan",
 		"google/" + google.Gemini15Flash,
+		"google/" + google.Gemini15FlashLatest,
 		"google/" + google.GeminiPro,
+		"google/" + google.GeminiProLatest,
 		"fireworks/accounts/sourcegraph/models/starcoder-7b",
 		"fireworks/accounts/sourcegraph/models/starcoder-16b",
 		"fireworks/accounts/fireworks/models/starcoder-3b-w8a16",