fix: use locale-independent formatting for purchase prices

- Fixes an issue where purchase amounts were being formatted using the device's locale settings, causing European locales to send "12,99" instead of "12.99". This caused 400 errors from the API and blocked all user property updates.
- The fix ensures the NSNumberFormatter uses en_US_POSIX locale, which always uses a period as the decimal separator regardless of device locale.

Author: nan-li

iOS_SDK/OneSignalSDK/Source/OneSignalTrackIAP.m

@@ -127,6 +127,7 @@ - (void)productsRequest:(id)request didReceiveResponse:(id)response {
 
             // SKProduct.price is an NSDecimalNumber, but the backend expects a String
             NSNumberFormatter *formatter = [NSNumberFormatter new];
+            [formatter setLocale:[NSLocale localeWithLocaleIdentifier:@"en_US_POSIX"]];
             [formatter setMinimumFractionDigits:2];
             NSString *formattedPrice = [formatter stringFromNumber:[skProduct performSelector:@selector(price)]];