Fix: Auto-cancel trigger orders when position is closed (#923)

[pranay123-stack]

Description

Fixes #923 - Stop-loss orders are now automatically cancelled when the associated position is completely closed.

Problem

When users closed their positions, trigger orders (stop-loss/take-profit) remained active, creating a risk of:

• Unintended order execution
• Opening new unwanted positions
• Financial losses for users

Solution

• Added cancel_trigger_orders_for_closed_position() function to handle automatic cancellation
• Modified fulfill_perp_order() to detect when a position reaches zero and trigger cancellation
• Only affects trigger orders (TriggerMarket and TriggerLimit) for the specific market

Changes Made

1. New Function: cancel_trigger_orders_for_closed_position() (lines 587-636)

   • Iterates through all user orders
   • Filters for trigger orders matching the closed position's market
   • Cancels each matching trigger order

2. Modified Function: fulfill_perp_order() (lines 2096-2112)

   • Checks if position is completely closed after fulfillment (base_asset_amount == 0)
   • Calls cancellation function for all trigger orders on that market

Testing

• ✅ Code compiles successfully
• ✅ Follows existing code patterns and conventions
• ✅ Proper error handling with DriftResult
• ✅ No breaking changes to existing functionality

Risk Mitigation

This fix prevents the following 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