feat(client): support reduce-only Perps orders

[cesarenaldi]

Summary

• add reduce_only to normal Perps order request and session placement APIs
• serialize reduce-only orders as ro: true
• expose reduce_only on parsed Perps orders from the ro response field
• add focused serialization, session, model, and typing coverage

Verification

• uv run pytest tests/unit/test_perps_trading_commands.py tests/unit/test_perps_session.py tests/unit/test_perps_events.py
• uv run ruff check
• uv run ruff format --check
• uv run pyright

[cursor]

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