Improve validations for MarketToLimitOrder

[nicolad]

• Adds new_checked (returns anyhow::Result<Self>) with full validation.

• Updates the existing new constructor to delegate to new_checked, preserving the old panic-on-failure behaviour for backwards-compatibility.

• Enforces:

  • quantity > 0
  • display_qty > 0 and display_qty ≤ quantity
  • expire_time is supplied when time_in_force == GTD

• Fills out the market_to_limit.rs doc-comment and removes outdated TODOs.

• Adds a dedicated test-suite (crates/model/src/orders/market_to_limit.rs) covering the happy path plus each new validation branch.

Rationale

Issue #2529 highlighted that MarketToLimitOrder lacked the strict invariants now applied to all order types.
Bringing these checks in-line:

• Eliminates a class of runtime panics only observed after order submission.
• Makes the validation surface symmetrical across Market-To-Limit, Limit-If-Touched, and Market-If-Touched orders, simplifying future maintenance.
• Ensures the builder-based test harnesses catch bad inputs early in CI.

Type of change

• Bug fix – strengthens existing validation without breaking the public API.

How has this change been tested?

• Added 4 rstest cases mirroring the patterns used in Improve validations for LimitIfTouchedOrder #2533/Improve validations for MarketIfTouchedOrder #2577:

  • ok (happy path)
  • quantity_zero
  • gtd_without_expire
  • display_qty_gt_quantity

Fixes #2529

[CLAassistant]

All committers have signed the CLA.