Add pre-operation validation hooks for mental model create/refresh

[cdbartholomew]

Summary

• Adds MentalModelRefreshContext dataclass for pre-operation validation context
• Adds validate_mental_model_refresh method to OperationValidatorExtension
• Wires up pre-operation validation in both create and refresh mental model HTTP routes
• Extensions can now reject mental model operations before async LLM work is queued

Changes

File	Change
hindsight-api/hindsight_api/extensions/operation_validator.py	Add MentalModelRefreshContext and validate_mental_model_refresh hook
hindsight-api/hindsight_api/extensions/__init__.py	Export MentalModelRefreshContext
hindsight-api/hindsight_api/api/http.py	Wire up pre-operation validation in create/refresh endpoints

Test plan

• Verify create mental model route calls validate_mental_model_refresh before creating
• Verify refresh mental model route calls validate_mental_model_refresh before refreshing
• Verify rejected operations return correct status code and reason
• Verify operations proceed normally when no validator is configured