MOBILE-160: minor cleanups around operationsDomain

- Configuration dartdoc: use Dart `null`/empty wording, remove the
  native-config reference that leaks SDK-internal behavior.
- MindboxAndroidPlugin: rename local to operationsDomainArg and apply
  the builder method outside the chain to avoid name collision with the
  builder method.
- example: drop the placeholder commented-out arg from the sample app.

Author: Vailence

mindbox_android/android/src/main/kotlin/cloud/mindbox/mindbox_android/MindboxAndroidPlugin.kt

@@ -72,14 +72,16 @@ class MindboxAndroidPlugin : FlutterPlugin, MethodCallHandler, ActivityAware, Ne
                     val previousInstallId: String = args["previousInstallationId"] as String
                     val subscribeIfCreated: Boolean = args["subscribeCustomerIfCreated"] as Boolean
                     val shouldCreateCustomer: Boolean = args["shouldCreateCustomer"] as Boolean
-                    val operationsDomain: String = args["operationsDomain"] as? String ?: ""
-                    val config = MindboxConfiguration.Builder(context.applicationContext, domain, endpointId)
+                    val operationsDomainArg: String = args["operationsDomain"] as? String ?: ""
+                    val builder = MindboxConfiguration.Builder(context.applicationContext, domain, endpointId)
                         .setPreviousDeviceUuid(previousDeviceUuid)
                         .setPreviousInstallationId(previousInstallId)
                         .subscribeCustomerIfCreated(subscribeIfCreated)
                         .shouldCreateCustomer(shouldCreateCustomer)
-                        .apply { if (operationsDomain.isNotEmpty()) operationsDomain(operationsDomain) }
-                        .build()
+                    if (operationsDomainArg.isNotEmpty()) {
+                        builder.operationsDomain(operationsDomainArg)
+                    }
+                    val config = builder.build()
                     Mindbox.init(activity = context, config, listOf())
                     result.success("initialized")
                 } else {

mindbox_platform_interface/lib/src/types/configuration.dart

@@ -15,8 +15,8 @@ class Configuration {
   /// Used for generating baseurl for REST.
   final String domain;
 
-  /// Optional host for sending operations. Overridden by
-  /// the value from the mobile JSON config when present. Default `nil` (use `domain`).
+  /// Optional separate host for sending operations. When empty, [domain]
+  /// is used. Default is an empty string.
   final String operationsDomain;
 
   /// Used for app identification on iOS.