Add per-call model override parameter to MCP tools

[aborruso]

Currently, the model used for image generation is fixed at server startup via the NANOBANANA_MODEL environment variable (falling back to gemini-2.5-flash-image). There is no way to switch models on a per-request basis without restarting the server.

Proposed change

Add an optional model parameter to all MCP tools (generate_image, edit_image, restore_image, generate_icon, generate_pattern, generate_story, generate_diagram). When provided, it overrides the server-level default for that single call.

Why it's useful

• The default flash model is fast and sufficient for most use cases
• For specific requests that need higher quality (e.g. gemini-3-pro-image-preview), the user can opt in per-call without changing server config
• No breaking change — when model is omitted, behaviour is identical to today

Implementation sketch

1. Add model?: string to ImageGenerationRequest in types.ts
2. In imageGenerator.ts, use request.model || this.modelName in each generateContent call
3. Expose the parameter in each tool's inputSchema in index.ts