Add model validation cache with 5-minute TTL to reject invalid model IDs
upfront instead of sending them to the API. This provides better error
messages and avoids unnecessary API calls.
- Add MODEL_VALIDATION_CACHE_TTL_MS constant (5 min)
- Add isValidModel() with lazy cache population
- Warm cache when listModels() is called
- Validate model ID in /v1/messages before processing
Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>