Refactor `_extract_size_bytes` method to prioritize primary model files when calculating size. The new implementation:
- Extracts size parsing into separate `parse_size` function
- Adds logic to prefer files marked as both "model" type and "primary"
- Falls back to first valid size if no primary model file found
- Adds comprehensive tests for primary preference and fallback behavior
This ensures more accurate size reporting for model files, particularly when multiple file types are present in the response.
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