Covers:
- Region + modelId correctly extracted for ap-northeast-1, us-east-1, us-west-2
- No region in path leaves modelId and optional_params unchanged
- Cross-region inference prefixes (us., eu., ap.) are not treated as region segments
- Explicitly set aws_region_name is not overridden by region in model path
Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>