fix: trade search Gateway endpoint

[rupurt]

Summary

Updates ProjectX.search_trades() to use the ProjectX Gateway trade search endpoint and request shape.

Root Cause

The SDK was calling GET /trades/search with query parameters. The Gateway API exposes trade history at POST /Trade/search with accountId, startTimestamp, and endTimestamp in the request body.

Changes

• Calls POST /Trade/search from search_trades().
• Sends Gateway timestamp fields in the request body.
• Parses the Gateway { success, trades } response shape while preserving support for list responses.
• Applies the existing limit argument client-side.
• Updates client trading tests and fixtures for the corrected endpoint and payload.

Scope

This PR is endpoint and request-shape only. It does not change trade model fields or position response handling.

Related PRs:

• fix: preserve gateway trade commissions & normalize trade response fields #87 preserves Gateway trade commissions in the Trade model.
• fix: handle position display fields #88 handles Gateway position contractDisplayName.

Validation

• uv run ruff check src/project_x_py/client/trading.py tests/conftest.py tests/client/test_trading.py tests/client/test_trading_legacy.py
• uv run pytest tests/client/test_trading.py tests/client/test_trading_legacy.py