Fix clear session bug

[two-tiger]

Cleaning up incomplete groups

• rl/buffer_server.py:315 Added TTL-based cleanup: if a pending group fails to collect the required number of completed sessions (group_size) within a certain timeframe, the incomplete group is discarded.
• rl/buffer_server.py:280 Added tracking for discarded groups to ensure that late-arriving database rows do not re-add them to the pending queue.
• rl/buffer_server.py:218 Notified the LLM proxy to clear relevant session_ids when a group is discarded, preventing stale session states from persisting in the trajectory mask builder.
• rl/llm_proxy.py:326 Added the /admin/clear_sessions management endpoint to handle batch session cleanup requests from the buffer server.
• rl/mask/trajectory_mask_builder.py:520 Added the clear_sessions() interface for batch cleanup.