fix: tolerate empty-string decimal fields in trade responses

[Envy-Life]

Problem

GET /data/trades responses can carry "" in Decimal-typed fields, which makes the whole trades fetch fail to deserialize:

invalid value: string "", expected a Decimal type representing a fixed-point number

We hit this in production while paging a user's trades through Client::trades — one trade with an empty decimal field poisons the entire page.

This is the same venue behavior already handled for TradeResponse.transaction_hash (empty until the async execution pipeline broadcasts the trade's transaction, tolerated via empty_string_as_zero_hash) and for PostOrderResponse.making_amount/taking_amount (empty_string_as_zero) — but the Decimal fields of TradeResponse and its MakerOrders still hard-fail on "".

Fix

Apply the existing empty_string_as_zero deserializer to:

• TradeResponse: size, fee_rate_bps, price
• MakerOrder: matched_amount, price, fee_rate_bps

No API change — only widens accepted wire input, mirroring the established pattern.

Testing

Added trades_should_tolerate_empty_decimal_fields (mirrors trades_should_tolerate_empty_transaction_hash), asserting all six fields deserialize to Decimal::ZERO when the venue sends "".

cargo test --features clob --test clob trades_should_
running 4 tests ... 4 passed

(Note: couldn't apply the wip label from a fork — maintainers, feel free to add if desired.)

---

Note

Low Risk
Deserialization-only widening with an established helper; no API or trading logic changes, though callers may see zero instead of a hard error for malformed venue data.

Overview
Trade list deserialization no longer fails when the venue sends "" for numeric trade fields on GET /data/trades.

TradeResponse (size, fee_rate_bps, price) and nested MakerOrder (matched_amount, price, fee_rate_bps) now use the existing empty_string_as_zero deserializer, matching PostOrderResponse and the empty transaction_hash handling. Empty strings map to Decimal::ZERO; valid numeric strings behave as before.

A CLOB integration test asserts a full trades page still parses when those six fields are empty on the trade and its maker order.

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