_get_max_cached_blocks truncates small positive KVCACHED_MAX_CACHED_TOKENS to the disabled

`kvcached/integration/vllm/patches.py:256` does `MAX_CACHED_TOKENS // block_size`. With e.g. `KVCACHED_MAX_CACHED_TOKENS=8` and `block_size=16`, this returns `0`. Result: prefix caching is silently turned off.

Fix: maybe we should add a low boundary like:
​```python
return max(1, MAX_CACHED_TOKENS // block_size)
​```