fix: update task.slot_mapping in _set_slot_mapping_impl

[xianweihuihuan]

Problem

In _set_slot_mapping_impl, task.graph GPU block ids and task.status
were updated, but task.slot_mapping was not.

This caused merge_to_batch_kvtask to read the fake zero slot_mapping
set during create_get_task, leading to incorrect batch KV task merging.

Fix

Add task.slot_mapping = slot_mapping after updating the graph,
ensuring the task reflects the real slot mapping before status is
set to READY.

[linhu-nv]

that's really a big issue. I am just curious why this is not exposed earlier to us. Anyway thanks for the fix.

[xianweihuihuan]

Thanks for the kind words! I noticed it while studying the codebase
for my own project. Happy to contribute!