🤖 feat: Gemini 3.1 Pricing and Context Window (#11884)

- Added support for the new Gemini 3.1 models, including 'gemini-3.1-pro-preview' and 'gemini-3.1-pro-preview-customtools'.
- Updated pricing logic to apply standard and premium rates based on token usage thresholds for the new models.
- Enhanced tests to validate pricing behavior for both standard and premium scenarios.
- Modified configuration files to include Gemini 3.1 models in the default model lists and token value mappings.
- Updated environment example file to reflect the new model options.

api/models/spendTokens.spec.js

@@ -878,6 +878,135 @@ describe('spendTokens', () => {
       expect(result.completion.completion).toBeCloseTo(-expectedCompletionCost, 0);
     });
 
+    it('should charge standard rates for gemini-3.1-pro-preview when prompt tokens are below threshold', async () => {
+      const initialBalance = 100000000;
+      await Balance.create({
+        user: userId,
+        tokenCredits: initialBalance,
+      });
+
+      const model = 'gemini-3.1-pro-preview';
+      const promptTokens = 100000;
+      const completionTokens = 500;
+
+      const txData = {
+        user: userId,
+        conversationId: 'test-gemini31-standard-pricing',
+        model,
+        context: 'test',
+        balance: { enabled: true },
+      };
+
+      await spendTokens(txData, { promptTokens, completionTokens });
+
+      const expectedCost =
+        promptTokens * tokenValues['gemini-3.1'].prompt +
+        completionTokens * tokenValues['gemini-3.1'].completion;
+
+      const balance = await Balance.findOne({ user: userId });
+      expect(balance.tokenCredits).toBeCloseTo(initialBalance - expectedCost, 0);
+    });
+
+    it('should charge premium rates for gemini-3.1-pro-preview when prompt tokens exceed threshold', async () => {
+      const initialBalance = 100000000;
+      await Balance.create({
+        user: userId,
+        tokenCredits: initialBalance,
+      });
+
+      const model = 'gemini-3.1-pro-preview';
+      const promptTokens = 250000;
+      const completionTokens = 500;
+
+      const txData = {
+        user: userId,
+        conversationId: 'test-gemini31-premium-pricing',
+        model,
+        context: 'test',
+        balance: { enabled: true },
+      };
+
+      await spendTokens(txData, { promptTokens, completionTokens });
+
+      const expectedCost =
+        promptTokens * premiumTokenValues['gemini-3.1'].prompt +
+        completionTokens * premiumTokenValues['gemini-3.1'].completion;
+
+      const balance = await Balance.findOne({ user: userId });
+      expect(balance.tokenCredits).toBeCloseTo(initialBalance - expectedCost, 0);
+    });
+
+    it('should charge premium rates for gemini-3.1-pro-preview-customtools when prompt tokens exceed threshold', async () => {
+      const initialBalance = 100000000;
+      await Balance.create({
+        user: userId,
+        tokenCredits: initialBalance,
+      });
+
+      const model = 'gemini-3.1-pro-preview-customtools';
+      const promptTokens = 250000;
+      const completionTokens = 500;
+
+      const txData = {
+        user: userId,
+        conversationId: 'test-gemini31-customtools-premium',
+        model,
+        context: 'test',
+        balance: { enabled: true },
+      };
+
+      await spendTokens(txData, { promptTokens, completionTokens });
+
+      const expectedCost =
+        promptTokens * premiumTokenValues['gemini-3.1'].prompt +
+        completionTokens * premiumTokenValues['gemini-3.1'].completion;
+
+      const balance = await Balance.findOne({ user: userId });
+      expect(balance.tokenCredits).toBeCloseTo(initialBalance - expectedCost, 0);
+    });
+
+    it('should charge premium rates for structured gemini-3.1 tokens when total input exceeds threshold', async () => {
+      const initialBalance = 100000000;
+      await Balance.create({
+        user: userId,
+        tokenCredits: initialBalance,
+      });
+
+      const model = 'gemini-3.1-pro-preview';
+      const txData = {
+        user: userId,
+        conversationId: 'test-gemini31-structured-premium',
+        model,
+        context: 'test',
+        balance: { enabled: true },
+      };
+
+      const tokenUsage = {
+        promptTokens: {
+          input: 200000,
+          write: 10000,
+          read: 5000,
+        },
+        completionTokens: 1000,
+      };
+
+      const result = await spendStructuredTokens(txData, tokenUsage);
+
+      const premiumPromptRate = premiumTokenValues['gemini-3.1'].prompt;
+      const premiumCompletionRate = premiumTokenValues['gemini-3.1'].completion;
+      const writeRate = getCacheMultiplier({ model, cacheType: 'write' });
+      const readRate = getCacheMultiplier({ model, cacheType: 'read' });
+
+      const expectedPromptCost =
+        tokenUsage.promptTokens.input * premiumPromptRate +
+        tokenUsage.promptTokens.write * writeRate +
+        tokenUsage.promptTokens.read * readRate;
+      const expectedCompletionCost = tokenUsage.completionTokens * premiumCompletionRate;
+
+      expect(result.prompt.prompt).toBeCloseTo(-expectedPromptCost, 0);
+      expect(result.completion.completion).toBeCloseTo(-expectedCompletionCost, 0);
+    });
+
     it('should not apply premium pricing to non-premium models regardless of prompt size', async () => {
       const initialBalance = 100000000;
       await Balance.create({