Function validateCommonSignal

Validates the common fields of ISignalDto that apply to both pending and scheduled signals.

Checks:

• position is "long" or "short"
• priceOpen, priceTakeProfit, priceStopLoss are finite positive numbers
• price relationships are correct for position direction (TP/SL on correct sides of priceOpen)
• TP/SL distance constraints from GLOBAL_CONFIG
• minuteEstimatedTime is valid

Does NOT check:

• currentPrice vs SL/TP (immediate close protection — handled by pending/scheduled validators)
• ISignalRow-specific fields: id, exchangeName, strategyName, symbol, _isScheduled, scheduledAt, pendingAt