fix: handle position display fields

[rupurt]

Summary

Adds support for ProjectX Gateway position payloads that include contractDisplayName and other additive fields.

Root Cause

search_open_positions() passed raw Gateway position payloads directly to Position(**pos). Gateway responses can include contractDisplayName, which was not modeled by the SDK and caused construction to fail when present.

Changes

• Adds contractDisplayName to the Position model.
• Adds contractDisplayName to the PositionResponse TypedDict.
• Parses position responses through the modeled Position fields so unknown future Gateway fields do not crash the SDK.
• Adds regression coverage for preserving contractDisplayName while ignoring an unknown Gateway field.

Scope

This PR is intentionally independent from the trade endpoint and trade response normalization PRs. It contains one commit on top of main and only touches position response handling.

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

[blurxy]

We hit this exact bug independently on 3.5.9: the ProjectX gateway returns additive fields (e.g. contractDisplayName) in position payloads, and Position(**payload) raises TypeError: unexpected keyword argument — which in a trading integration silently strips the broker-side stop from every entry (the add_stop_loss/list_positions path dies). Routing payloads through the modeled fields + ignoring unknowns (as this PR does) is the right fix and hardens against the next additive field too. +1 to merge. (We're working around it downstream by wrapping Position.__init__ to tolerate unknown kwargs.)