feat: add per-call model override to MCP tools

[nexivionlabs]

Summary

Closes #44. Adds an optional model parameter to all 7 MCP tools so a single call can override the server-default model (NANOBANANA_MODEL / DEFAULT_MODEL) without restarting the server. Omitting it preserves the current behavior (purely additive, backward-compatible).

Changes

• types.ts: optional model?: string on ImageGenerationRequest.
• imageGenerator.ts: a small resolveModel(request) helper (request.model || this.modelName) used at all three generateContent call sites (generateTextToImage, generateStorySequence, editImage); a DEBUG log is emitted only when an override is actually applied.
• index.ts: a model property added to all 7 tool input schemas (optional, not required) and threaded into each ImageGenerationRequest in the call handler.

Testing

• npm run build / tsc --noEmit: pass.
• npm run lint:ci (eslint, --max-warnings 0): pass.
• Backward compatible: omitting model reproduces the existing behavior exactly; the override falls back to the server default when unset.