Improve validations for LimitIfTouchedOrder

[nicolad]

Standardized validations for LimitIfTouchedOrder (fixes #2529)

Summary of the changes

• Implemented new_checked for LimitIfTouchedOrder, bringing it in line with MarketOrder and LimitOrder.
• Added complete domain‑level validations:
  • positive quantity, price, trigger_price
  • optional display_qty ≤ quantity
  • TimeInForce::Gtd → non‑zero expire_time required
  • side/trigger rule (BUY ⇒ trigger ≥ limit, SELL ⇒ trigger ≤ limit)
• Legacy new now wraps new_checked, preserving the old panic‑on‑error behaviour for external callers.
• Added a concise Display implementation.
• Unit‑tests (4) live in the same module and cover happy‑path plus every validation failure.
• Inline rust‑doc comments for clarity.

Type of change

• Bug fix (non‑breaking change which fixes an issue)
• New feature (non‑breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)
• This change requires a documentation update

How has this change been tested?

• cargo test -p nautilus-model --lib limit_if_touched --features python – rule set runs 5 dedicated unit-tests:
  • ok
  • quantity_zero
  • gtd_without_expire
  • buy_trigger_lt_price
  • sell_trigger_gt_price

[CLAassistant]

All committers have signed the CLA.

[cjdsellers]

Thanks so much for the contribution!

Just a couple of small comments to address, then we can merge this 👌.