🤖 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/tx.js

@@ -200,6 +200,7 @@ const tokenValues = Object.assign(
     'gemini-2.5-flash-image': { prompt: 0.15, completion: 30 },
     'gemini-3': { prompt: 2, completion: 12 },
     'gemini-3-pro-image': { prompt: 2, completion: 120 },
+    'gemini-3.1': { prompt: 2, completion: 12 },
     'gemini-pro-vision': { prompt: 0.5, completion: 1.5 },
     grok: { prompt: 2.0, completion: 10.0 }, // Base pattern defaults to grok-2
     'grok-beta': { prompt: 5.0, completion: 15.0 },
@@ -330,6 +331,8 @@ const cacheTokenValues = {
   'kimi-k2-0711-preview': { write: 0.6, read: 0.15 },
   'kimi-k2-thinking': { write: 0.6, read: 0.15 },
   'kimi-k2-thinking-turbo': { write: 1.15, read: 0.15 },
+  // Gemini 3.1 models - cache read: $0.20/1M (<=200k), cache write: standard input price
+  'gemini-3.1': { write: 2, read: 0.2 },
 };
 
 /**
@@ -340,6 +343,7 @@ const cacheTokenValues = {
 const premiumTokenValues = {
   'claude-opus-4-6': { threshold: 200000, prompt: 10, completion: 37.5 },
   'claude-sonnet-4-6': { threshold: 200000, prompt: 6, completion: 22.5 },
+  'gemini-3.1': { threshold: 200000, prompt: 4, completion: 18 },
 };
 
 /**