fix: sort-agnostic orderbook walk for market price calculation

[deipeypaanchal]

Summary

• calculate_buy_market_price / calculate_sell_market_price iterated reversed(positions), assuming the server returns asks descending and bids ascending. That's an undocumented contract. If the server ever changes ordering (or a caller passes a pre-sorted snapshot), the walk returns the worst price rather than the marginal price required to fill the amount.
• Sort locally instead:
  • buys walk asks ascending (cheapest first)
  • sells walk bids descending (highest first)
• Added a _normalize_positions helper to consolidate the dict-vs-dataclass access.

Test plan

• BUY walk returns the same marginal price for ascending and descending ask inputs.
• SELL walk returns the same marginal price for ascending and descending bid inputs.
• FOK with insufficient book still raises.
• FAK with insufficient book returns the worst price walked.

🤖 Generated with Claude Code

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Note

Medium Risk
Changes the core market-price calculation used for building market orders, which can alter execution price selection and edge-case behavior when books are partially fillable.

Overview
Fixes market price estimation to be independent of server/orderbook input ordering.

calculate_buy_market_price and calculate_sell_market_price now normalize position entries (dict vs object), locally sort by price (asks ascending, bids descending), and return the marginal price needed to fill amount_to_match; for non-FOK orders with insufficient depth, they return the worst price reached during the walk instead of relying on the first entry.

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