test(models): add unit tests for model.ts

[ANSHSINGH050404]

Adds 38 unit tests for `src/models.ts` covering all provider resolution paths.

Closes #73

What's tested

Group	Tests	Coverage
Missing API keys	10	ConfigurationError for every provider + gateway combo
Direct routing	6	Google, Anthropic, OpenAI + alias resolution + unknown provider
Vercel gateway	2	Routes through gateway(), missing key
OpenRouter gateway	3	Routes through OpenRouter, alias map
OpenCodeZen gateway	6	Prefix stripping, all OPENCODEZEN_MODEL_ALIASES
Cloudflare gateway	5	CF Google/Anthropic routing, alias resolution, unsupported provider
Gateway override	3	Override param precedence, fallbacks
Model ID parsing	3	Multi-slash, provider-only, single-slash IDs