Add Gemini 3 support, fix tool call signatures, and add CORS middleware

[nurujjamanpollob]

Description

This PR introduces core support for the new Gemini 3 models, alongside several critical server enhancements to improve tool-calling reliability and browser-based client compatibility.

🚀 Key Changes

• ✨ Gemini 3 Models Support: Added full mappings and compatibility for the new gemini-3-pro-preview and gemini-3-flash-preview models.
• 🔧 Tool Call Signature Fix: Resolved payload formatting issues during tool execution to correctly match OpenAI/Anthropic spec expectations, preventing malformed requests.
• 🌐 CORS Middleware (fix(server)): Integrated CORS middleware into the Express server to prevent Cross-Origin Resource Sharing errors for directly connected local web apps and browser clients.

🧪 How to Test

1. Start the server via npm run dev.
2. Route a request through the proxy using a Gemini 3 model alias (e.g., gemini-3-pro-high).
3. Trigger a functional tool call from an agent (e.g., Claude Code, Aider) and verify the proxy console logs and proper signature interpretation.
4. Make an API request from a local browser environment to http://localhost:3456 to confirm CORS headers are correctly applied and requests are not blocked.