feat(cody-gateway): add support for Gemini models with context cache (#63413)

PART OF https://linear.app/sourcegraph/issue/CODY-2451
CLOSE https://linear.app/sourcegraph/issue/CODY-2513

- Add Gemini 1.5 Flash 001 and Gemini 1.5 Pro 001 models to the config
and allowed models lists
- These fixed stable versions support context caching, as noted in the
[Google Gemini API
docs](https://ai.google.dev/gemini-api/docs/caching?lang=node)


![image](https://github.com/sourcegraph/sourcegraph/assets/68532117/1857c853-7c8d-4446-a991-4bc6a39e6065)


NEXT: Implement context caching in the codebase. Right now using the
newly added models alone do not work with context caching.

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## Test plan

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No feature changes. Adding new model to allow list.

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cmd/cody-gateway/shared/config/config.go

@@ -311,6 +311,8 @@ func (c *Config) Load() {
 			google.Gemini15FlashLatest,
 			google.Gemini15ProLatest,
 			google.GeminiProLatest,
+			google.Gemini15Flash001,
+			google.Gemini15Pro001,
 			google.Gemini15Flash,
 			google.Gemini15Pro,
 			google.GeminiPro,

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

@@ -335,6 +335,7 @@ var allCodeCompletionModels = slices.Concat([]string{"anthropic/" + anthropic.Cl
 	"anthropic/claude-instant-1.2",
 	"google/" + google.Gemini15Flash,
 	"google/" + google.Gemini15FlashLatest,
+	"google/" + google.Gemini15Flash001,
 	"google/" + google.GeminiPro,
 	"google/" + google.GeminiProLatest,
 	"fireworks/starcoder",
@@ -377,6 +378,8 @@ func allowedModels(scope types.CompletionsFeature, isProUser bool) []string {
 			"google/" + google.Gemini15FlashLatest,
 			"google/" + google.Gemini15ProLatest,
 			"google/" + google.GeminiProLatest,
+			"google/" + google.Gemini15Flash001,
+			"google/" + google.Gemini15Pro001,
 			"google/" + google.Gemini15Flash,
 			"google/" + google.Gemini15Pro,
 			"google/" + google.GeminiPro,

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

@@ -100,6 +100,7 @@ func isAllowedCodeCompletionModel(model string) bool {
 		"anthropic/claude-instant-1.2-cyan",
 		"google/" + google.Gemini15Flash,
 		"google/" + google.Gemini15FlashLatest,
+		"google/" + google.Gemini15Flash001,
 		"google/" + google.GeminiPro,
 		"google/" + google.GeminiProLatest,
 		"fireworks/accounts/sourcegraph/models/starcoder-7b",
@@ -132,6 +133,8 @@ func isAllowedCustomChatModel(model string, isProUser bool) bool {
 			"google/" + google.Gemini15FlashLatest,
 			"google/" + google.Gemini15ProLatest,
 			"google/" + google.GeminiProLatest,
+			"google/" + google.Gemini15Flash001,
+			"google/" + google.Gemini15Pro001,
 			"google/" + google.Gemini15Flash,
 			"google/" + google.Gemini15Pro,
 			"google/" + google.GeminiPro,

internal/completions/client/google/models.go

@@ -19,3 +19,9 @@ const GeminiPro = "gemini-pro"
 const Gemini15FlashLatest = "gemini-1.5-flash-latest"
 const Gemini15ProLatest = "gemini-1.5-pro-latest"
 const GeminiProLatest = "gemini-pro-latest"
+
+// Fixed stable versions
+// NOTE: Only the fixed stable versions support context caching.
+// Ref: https://ai.google.dev/gemini-api/docs/caching?lang=node
+const Gemini15Flash001 = "gemini-1.5-flash-001"
+const Gemini15Pro001 = "gemini-1.5-pro-001"