fix: Make initialDelaySeconds optional in iOS registerOneOffTask

The iOS native implementation was requiring initialDelaySeconds as a
non-null Int64, but the Dart API allows it to be optional. This was
causing "Invalid parameters passed" errors in integration tests.

Changed to use a default value of 0 when the parameter is not provided,
matching the behavior of registerPeriodicTask and registerProcessingTask.

🤖 Generated with [Claude Code](https://claude.ai/code)

Co-Authored-By: Claude <[email protected]>

Author: ened

workmanager_ios/ios/Classes/SwiftWorkmanagerPlugin.swift

@@ -310,11 +310,7 @@ extension SwiftWorkmanagerPlugin: FlutterPlugin {
 
         if #available(iOS 13.0, *) {
             let method = ForegroundMethodChannel.Methods.RegisterOneOffTask.self
-            guard let delaySeconds =
-                arguments[method.Arguments.initialDelaySeconds.rawValue] as? Int64 else {
-                result(WMPError.invalidParameters.asFlutterError)
-                return
-            }
+            let delaySeconds = arguments[method.Arguments.initialDelaySeconds.rawValue] as? Int64 ?? 0
             guard let uniqueTaskIdentifier =
                 arguments[method.Arguments.uniqueName.rawValue] as? String else {
                 result(WMPError.invalidParameters.asFlutterError)