Add comprehensive tests for Assets API (14 tests)
Tests include:
- Asset model validation (8 tests)
- Asset, AssetRename, FolderCreate models
- Field validation, filename validation
- Size helper methods (size_mb, size_kb)
- Slug normalization
- Sync AssetsEndpoint (6 tests)
- List, get, rename, delete operations
- Folder listing
- Validation error handling
All 14 tests passing. Achieves comprehensive coverage for Assets API.
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Co-Authored-By: Claude <noreply@anthropic.com>
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