[BugFix][KVCache] fix gpu_free_block_list returning wrong block IDs

## Motivation

`gpu_free_block_list` 的兼容 property 中误用了 `list(range(N))`，
将 `available_blocks()` 的返回值当作整数传给 `range()`，
导致返回 `[0, 1, ..., N-1]` 的假列表，而非真实的空闲 block ID。

## Modifications

- `cache_manager/v1/cache_manager.py`：将 `list(range(self._device_pool.available_blocks()))` 改为 `list(self._device_pool.available_blocks())`

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

Author: kevincheng2

fastdeploy/cache_manager/v1/cache_manager.py

@@ -426,7 +426,7 @@ def gpu_free_block_list(self) -> List[int]:
         with PrefixCacheManager.gpu_free_block_list.
         """
         # Return list representation of available blocks
-        return list(range(self._device_pool.available_blocks()))
+        return list(self._device_pool.available_blocks())
 
     @property
     def available_gpu_resource(self) -> float: