refund exact amounts on order cancellation

[Tomoya0k]

Closes #268.

Summary

• add regression coverage for exact order-cancellation refunds in tests/order_cancellation.rs
• verify user-initiated cancellation returns the exact collateral amount and restores the execution fee
• verify keeper-initiated cancellation after failed execution returns exact collateral to the user and pays the execution fee to the keeper
• verify partial collateral cancellation refunds the exact deposited amount with no rounding loss
• fix order_handler::cancel_order so execution_fee is paid out on cancellation instead of remaining stuck in the vault

Tests

• cargo test -p gmx-integration-tests --test order_cancellation
• cargo test -p order-handler cancel_order

Notes

• The test environment uses mocked auth and does not charge native transaction fees, so XLM refund assertions expect exact restoration with no fee offset.
• cargo test -p gmx-integration-tests still has unrelated pre-existing failures in other integration tests and was not fully green before this change.