feat: Add dedicated condensation provider/model configuration

- Add 'condensation' section to providers.json for specifying dedicated provider/model
- Add CondensationConfig model to config.py
- Add _load_condensation() and get_condensation() methods
- Update ContextManager to use dedicated condensation handler when configured
- Update handlers to pass condensation config to ContextManager
- Allows using smaller/faster model for context condensation operations

This addresses the issue where conversational and semantic condensation
methods were using the same model as the main request, which was
inefficient. Now users can configure a dedicated provider and model
for condensation operations, typically using a smaller/faster model to reduce
costs and improve performance.