Feature: MARKET variant + auto-price for order convenience functions

[MDUYN]

Description

Follow-up to #440 / #555. The convenience helpers (order_value, order_percent, order_target, order_target_value, order_target_percent) currently require an explicit price argument and always produce LIMIT orders. Extend them so users can omit price and/or request a MARKET order.

Desired Behavior

1. Auto-price: When price is omitted, fall back to context.get_latest_price(target_symbol, market) to size the order — same mechanism create_market_order already uses for percentage_of_portfolio sizing.
2. MARKET variant: Accept order_type=OrderType.MARKET on all five helpers. When set, route through create_market_order (no price required; uses the estimated latest price internally and fills at next-bar open in backtests).
3. Validation: If order_type=OrderType.LIMIT and price is None and get_latest_price returns None, raise OperationalException with a clear message.

Suggested API

# LIMIT with explicit price (today's behavior — unchanged)
context.order_target_percent("BTC", target_percent=10, price=price)

# LIMIT, auto-price from data provider
context.order_target_percent("BTC", target_percent=10)

# MARKET (immediate fill at next-bar open in backtests)
context.order_target_percent(
    "BTC", target_percent=10, order_type=OrderType.MARKET
)

Notes

• The five helpers are thin wrappers around create_limit_order today; for MARKET routing they should delegate to create_market_order instead.
• For order_target*, the diff (BUY/SELL amount) is still computed against the current position; only the fill path changes.
• All new behavior should be covered by tests in tests/app/algorithm/test_order_convenience_functions.py.

Related

• Implemented in feat(context): order convenience functions (#440) #555: LIMIT-only with explicit price.
• Closes the original issue Feature: Order Convenience Functions (order_value, order_percent, order_target) #440.