Evict migrated blocks from prefix cache after KV send completes

After a session migrates from C to D via offload, C's blocks were freed
to the LRU tail (most-recently-used position), making them the last to
be evicted. Since the session won't return to C, these blocks are dead
weight occupying cache capacity.

Now capture block IDs before _free_blocks and call evict_blocks to
remove them from the prefix cache hash table, so they can be reused
sooner for active sessions.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

third_party/vllm/vllm/v1/core/sched/scheduler.py

@@ -2116,7 +2116,12 @@ class Scheduler(SchedulerInterface):
             if req_id not in self.requests:
                 logger.warning("Skipping finished_sending for unknown request %s (already aborted?)", req_id)
                 continue
+            sent_block_ids: set[int] = set()
+            for group in self.kv_cache_manager.get_block_ids(req_id):
+                sent_block_ids.update(group)
             self._free_blocks(self.requests[req_id])
+            if sent_block_ids:
+                self.kv_cache_manager.evict_blocks(sent_block_ids)
 
     def _update_requests_with_invalid_blocks(
         self,