Normalize taker and maker amounts in orders

[gmoutsin]

When creating an order, the price is multiplied by the size. Often rust overestimates the scale of the product. This can lead to decimal number such as 156730010^(-5) instead of 1567310^(-3) and subsequently this may be rejected by the server. Normalizing the amounts before creating the order solves this problem.

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Note

Low Risk
Two-line normalization in limit order build only; intended numeric values unchanged, reduces spurious server rejections.

Overview
Limit order construction now canonicalizes taker_amount and maker_amount with normalize_assign() right after size * price is truncated, and before those values are passed through to_fixed_u128 into the order payload.

That addresses cases where rust_decimal keeps an inflated scale on the product (same value, different representation), which the CLOB server can reject even though to_fixed_u128 already normalizes at encode time.

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