test(core): add tests for Configuration and ConfigurationModelABC
- ConfigurationModelABC: defaults, src parsing, PascalCase/snake_case key variants, type casting, required fields, readonly enforcement, env var override with prefix - Configuration: set/get by class and string key, auto-instantiation of unregistered models, add_json_file loading, optional/missing files Also documents two additional bugs found: - cast(True, bool) fails with AttributeError (bool has no .lower()) - add_json_file does not exit on invalid JSON (swallows parse error) Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>
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