deepbook: add cancel_live_order(s) graceful cancel APIs (DBU-349)

[tonylee08]

Summary

• Add cancel_live_order(order_id) and cancel_live_orders(order_ids) on Pool. Both no-op when an id is not in the balance manager's account_open_orders (already filled, cancelled, expired-and-swept, or not owned by this BM), instead of aborting with big_vector::ENotFound like cancel_order / cancel_orders do today.
• Existing cancel_order / cancel_orders are unchanged — graceful behavior is opt-in via the new functions, no ABI break.
• Linear: DBU-349

Key decisions

• Authorization stays strict. Orders in account.open_orders() belong to that BM by construction (state/account.move:161), so the live path still routes through cancel_order and its EInvalidOrderBalanceManager assert. An id owned by a different BM falls through the contains check and is silently skipped — matches the graceful intent without weakening any guard.
• Local snapshot for the batch variant. cancel_live_orders snapshots account_open_orders once and removes ids from the local copy as it cancels them. This handles duplicate ids in the input vector naturally — the second occurrence misses the local contains check instead of aborting in the underlying cancel_order.
• Expired-but-not-swept orders are still cancelled. If an order is past its expiry but still in account.open_orders(), the graceful variants route it through cancel_order (which handles expiry inside process_cancel). They do not silently drop expired orders.
• No new error codes / no new events. Skipped ids are silent — no OrderCanceled event is emitted for them. Successful cancels emit the existing OrderCanceled event via the unchanged cancel_order path.

Test plan

• cancel_live_order_succeeds_for_live_order — happy path on a single live order
• cancel_live_order_skips_filled_order — single id that was filled by a crossing taker is silently skipped
• cancel_live_order_noop_for_unknown_id — never-placed id is a no-op
• cancel_live_orders_skips_filled_in_batch — mixed batch (one filled, one live); filled is skipped, live is cancelled
• cancel_live_orders_handles_duplicate_ids — [id, id] does not abort on the second pass
• Existing cancel-family suite (23 tests) still green: test_cancel_orders_*, test_cancel_all_orders_*, test_place_and_cancel_*, test_self_matching_cancel_*

🤖 Generated with Claude Code

[pei-mysten]

LGTM