fix: preserve gateway trade commissions & normalize trade response fields

[rupurt]

Summary

Makes search_trades() resilient to Gateway trade payload variants while preserving Gateway commission data.

Root Cause

Some Gateway trade search responses return commissions instead of fees, and may include additional fields not accepted by the SDK Trade model. Passing the raw payload directly to Trade(**trade) can raise TypeError, and mapping commissions only into fees would hide a Gateway field callers may need.

Changes

• Adds optional Trade.commissions.
• Adds optional commissions to TradeResponse.
• Keeps existing fees compatibility by populating fees from commissions when fees is absent.
• Ignores unknown future response fields that are not part of the Trade model.
• Adds regression coverage for preserving Gateway commissions.

Scope

This PR is trade response normalization and model support only. It does not change the trade search endpoint or position response handling.

Related PRs:

• fix: trade search Gateway endpoint #86 fixes the Gateway trade search endpoint and request shape.
• fix: handle position display fields #88 handles Gateway position contractDisplayName.

Validation

• uv run pytest tests/client/test_trading.py tests/client/test_trading_legacy.py tests/types/test_models.py tests/types/test_api_responses.py -q
• uv run ruff check src/project_x_py/client/trading.py src/project_x_py/models.py src/project_x_py/types/api_responses.py tests/client/test_trading_legacy.py tests/types/test_models.py tests/types/test_api_responses.py
• uv run ruff format --check src/project_x_py/client/trading.py src/project_x_py/models.py src/project_x_py/types/api_responses.py tests/client/test_trading_legacy.py tests/types/test_models.py tests/types/test_api_responses.py
• uv run mypy src/project_x_py/client/trading.py src/project_x_py/models.py src/project_x_py/types/api_responses.py