Add dataclasses and hook methods to OperationValidatorExtension for
tracking mental model operations:
- MentalModelGetContext/Result: context and result for GET operations
- MentalModelRefreshResult: result for refresh operations with token counts
- validate_mental_model_get: pre-operation validation hook
- on_mental_model_get_complete: post-GET completion hook
- on_mental_model_refresh_complete: post-refresh completion hook
Invoke hooks in http.py (GET endpoint) and memory_engine.py (refresh).
Add tests verifying hooks are called with correct parameters.
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