Fix: Auto-cancel trigger orders when position is closed

Resolves issue #923 where stop-loss (trigger) orders were not being
automatically cancelled when a user closed their position.

Changes:
- Added new function `cancel_trigger_orders_for_closed_position()` that
  cancels all trigger orders (TriggerMarket and TriggerLimit) for a
  specific market when called
- Modified `fulfill_perp_order()` to check if a position is completely
  closed after fulfillment (base_asset_amount == 0)
- If position is closed, automatically cancel all trigger orders for
  that market to prevent unintended order execution

This prevents the following risk scenario:
1. User opens a short position with a stop-loss order
2. User manually closes the short position
3. Previously: SL order remained active and could execute unexpectedly
4. Now: SL order is automatically cancelled when position closes

Author: pranay-hft

programs/drift/src/controller/orders.rs

@@ -584,6 +584,57 @@ pub fn cancel_orders(
     Ok(canceled_order_ids)
 }
 
+pub fn cancel_trigger_orders_for_closed_position(
+    user: &mut User,
+    user_key: &Pubkey,
+    market_index: u16,
+    market_type: MarketType,
+    perp_market_map: &PerpMarketMap,
+    spot_market_map: &SpotMarketMap,
+    oracle_map: &mut OracleMap,
+    now: i64,
+    slot: u64,
+) -> DriftResult {
+    for order_index in 0..user.orders.len() {
+        if user.orders[order_index].status != OrderStatus::Open {
+            continue;
+        }
+
+        // Only cancel trigger orders (TriggerMarket or TriggerLimit)
+        if !user.orders[order_index].must_be_triggered() {
+            continue;
+        }
+
+        // Only cancel orders for the specified market
+        if user.orders[order_index].market_type != market_type {
+            continue;
+        }
+
+        if user.orders[order_index].market_index != market_index {
+            continue;
+        }
+
+        cancel_order(
+            order_index,
+            user,
+            user_key,
+            perp_market_map,
+            spot_market_map,
+            oracle_map,
+            now,
+            slot,
+            OrderActionExplanation::OrderExpired,
+            None,
+            0,
+            false,
+        )?;
+    }
+
+    user.update_last_active_slot(slot);
+
+    Ok(())
+}
+
 pub fn cancel_order_by_order_id(
     order_id: u32,
     user: &AccountLoader<User>,
@@ -2042,6 +2093,24 @@ fn fulfill_perp_order(
         )?;
     }
 
+    // Cancel all trigger orders if position is completely closed
+    if base_asset_amount > 0 {
+        let position_index = get_position_index(&user.perp_positions, market_index)?;
+        if user.perp_positions[position_index].base_asset_amount == 0 {
+            cancel_trigger_orders_for_closed_position(
+                user,
+                user_key,
+                market_index,
+                MarketType::Perp,
+                perp_market_map,
+                spot_market_map,
+                oracle_map,
+                now,
+                slot,
+            )?;
+        }
+    }
+
     Ok((base_asset_amount, quote_asset_amount))
 }