[KVCache][BugFix] fix increment ref count logic in cache_manager

## Motivation

`increment_ref_nodes` should only be called during the scheduling
phase (when `skip_storage=True`), not during the actual storage
prefetch phase. The previous condition was inverted, causing ref
counts to be incremented at the wrong time.

## Modifications

- Fix condition from `not (self._storage_scheduler and skip_storage)`
  to `skip_storage` in `CacheManager.match`
- Update comment to clarify "only scheduling phase"

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

Author: kevincheng2

fastdeploy/cache_manager/v1/cache_manager.py

@@ -532,8 +532,8 @@ def match_prefix(
                     storage_matches = self._match_storage(remaining_hashes)
                     result.storage_nodes = self.prepare_prefetch_metadata(storage_matches)
 
-                # Step 3: Increment ref count for matched blocks(only first match node)
-                if not (self._storage_scheduler and skip_storage):
+                # Step 3: Increment ref count for matched blocks(only scheduling phase)
+                if skip_storage:
                     self._radix_tree.increment_ref_nodes(matched_nodes)
 
                 logger.info(