Enable fractional LongBridge orders

[Pigbibi]

Summary

• add LongBridge env controls for fractional quantity steps and min notional
• size sells/buys with quantity steps instead of hard integer shares
• preserve raw LongBridge position diagnostics behind LONGBRIDGE_DEBUG_POSITION_SNAPSHOT
• avoid blocking fractional buys when broker max-purchase estimate returns whole-share zero

Dependencies

• QuantStrategyLab/QuantPlatformKit fractional quantity helper PR
• QuantStrategyLab/UsEquityStrategies fractional strategy position PR

Tests

• ./.venv/bin/python -m pytest tests/test_runtime_config_support.py tests/test_rebalance_service.py tests/test_runtime_composer.py tests/test_runtime_broker_adapters.py tests/test_strategy_runtime.py

[chatgpt-codex-connector]

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[chatgpt-codex-connector]

Reject non-positive order quantity step from environment

_quantity_step_env returns the explicit LONGBRIDGE_ORDER_QUANTITY_STEP value without validation, so values like 0 or negative numbers are accepted and flow into execution sizing. In fractional/whole-share sizing paths this step is passed to floor_to_quantity_step(...), which can produce invalid quantities or raise at runtime (for example when a misconfigured deployment sets 0). Validate that the resolved step is finite and strictly greater than zero (and fail fast with a clear config error).

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