fix(clob): tolerate empty-string Decimals and missing tx_hash when deserializing fills

[popoxhb]

Problem

The live Polymarket CLOB API occasionally serializes Decimal money fields as empty strings ("") and omits transaction_hash for maker fills that are not yet on-chain. With the current strict typing this makes the entire /trades response batch fail to deserialize — a single "" in one record fails the whole Page<TradeResponse> parse, silently dropping every fill in that batch.

Observed in production on GET /trades:

• TradeResponse.size, TradeResponse.fee_rate_bps, TradeResponse.price → sometimes ""
• MakerOrder.matched_amount, MakerOrder.price, MakerOrder.fee_rate_bps → sometimes ""
• TradeResponse.transaction_hash → absent for maker fills not yet settled on-chain

Fix

• Apply the existing empty_string_as_zero deserializer (already used on PostOrderResponse.making_amount / taking_amount) to the six Decimal fields above, so "" parses as Decimal::ZERO instead of erroring.
• Mark TradeResponse.transaction_hash as #[serde(default)], so a missing hash deserializes to B256::default() (the zero hash) rather than failing.

Compatibility

Backward compatible: well-formed responses with present, non-empty values deserialize exactly as before. Only the previously-fatal empty/missing cases change — from "whole batch errors out" to "that field is zero / default". empty_string_as_zero already exists in response.rs, so this is attribute-only — no new helpers and no API/signature changes.

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Note

Low Risk
Attribute-only serde changes on response DTOs; no API or logic changes, with backward-compatible parsing for well-formed payloads.

Overview
Hardens /trades deserialization so a single malformed field no longer fails an entire Page<TradeResponse> batch from the live CLOB API.

On TradeResponse, size, fee_rate_bps, and price now use the existing empty_string_as_zero deserializer (same pattern as PostOrderResponse), so "" becomes Decimal::ZERO. transaction_hash gets #[serde(default)], so omitted hashes (e.g. maker fills not yet on-chain) deserialize to the default B256 instead of erroring.

On MakerOrder, matched_amount, price, and fee_rate_bps get the same empty_string_as_zero treatment. Normal non-empty values behave unchanged; only previously fatal empty/missing wire shapes are relaxed.

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