Add cache usage property to ChatSampler and Sampler

## Cache Usage Monitoring Enhancement

### Overview
Previously, a TODO was marked to add a property showing how much of the cache is used. This is useful for debugging and monitoring cache usage during multi-turn conversations.

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### Previous State
- No easy way to check cache usage without accessing internal `last_state` directly
- Users had to manually compute `state.used_cache_length / cache_length`

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### Changes

#### Added Cache Usage Monitoring
Cache usage monitoring was added to both sampler classes:

- **`ChatSampler.cache_usage` property**
- Returns a `(used, total)` tuple
- Returns `None` if no sampling has been performed yet

- **`Sampler.get_cache_usage()` static method — For stateless usage with the low-level API**
- Takes a SamplingState and cache_length as parameters