fix(broker_gateway): safe cTrader volume conversion with overflow checks

The previous (quantity * 100_000.0) as i64 silently truncated
fractional lots and could overflow on extreme values. Added
convert_quantity_to_volume() that validates the result is finite,
non-negative, and within i64 range, using round() instead of
truncation. Includes 7 unit tests covering normal values, fractional
rounding, overflow, negative, NaN, infinity, and zero edge cases.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

services/broker_gateway_service/src/service.rs

@@ -98,6 +98,7 @@ impl BrokerGatewayService {
 ///
 /// Returns an error if the result is not finite, negative, or exceeds i64 range.
 /// Uses rounding to avoid silent truncation of fractional lots.
+#[cfg_attr(not(feature = "icmarkets"), allow(dead_code))]
 fn convert_quantity_to_volume(quantity: f64) -> Result<i64, Status> {
     let volume_f = quantity * 100_000.0;
     if !volume_f.is_finite() || volume_f < 0.0 || volume_f > i64::MAX as f64 {