fix(order): keep order test on public headers

[SebastianBoehler]

Summary

• update order_test.cpp to use the public ClobClient order path instead of the private order_execution.hpp helper header
• let CreateMarketOrderParams accept cached neg_risk, matching CreateOrderParams and avoiding an extra lookup in the sample
• bump patch version/docs to v1.2.5 for the consumer-facing fix

Verification

• cmake --build /tmp/polymarket-issue20-build --target order_test test_order_execution -j 8
• ctest --test-dir /tmp/polymarket-issue20-build --output-on-failure
• installed package smoke: order_signer.hpp exists, order_execution.hpp remains private, public-header consumer compile passes